Added example using the library as a cmake external project.

This closes #29

examples/cmake_external_project/CMakeLists.txt

@@ -0,0 +1,32 @@
+cmake_minimum_required(VERSION 3.0)
+project (example)
+
+find_package(ZLIB REQUIRED)
+find_package(OpenSSL REQUIRED)
+find_package(Boost REQUIRED COMPONENTS
+    system
+    program_options
+    filesystem
+    date_time
+    context
+    coroutine
+    chrono
+    log
+    )
+
+include(cmake/external-projects.cmake)
+
+add_executable(${PROJECT_NAME} main.cpp)
+add_dependencies(${PROJECT_NAME} externalRestcCpp)
+set_property(TARGET ${PROJECT_NAME} PROPERTY CXX_STANDARD 14)
+target_link_libraries(${PROJECT_NAME}
+    restc-cpp
+    ${Boost_LIBRARIES}
+    ${ZLIB_LIBRARIES}
+    ${OPENSSL_LIBRARIES}
+    )
+
+add_definitions(
+    -DBOOST_COROUTINE_NO_DEPRECATION_WARNING=1
+    -DBOOST_ALL_DYN_LINK=1
+    )

examples/cmake_external_project/cmake/external-projects.cmake

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+# Here are registered all external projects
+#
+# Usage:
+# add_dependencies(TARGET externalProjectName)
+# target_link_libraries(TARGET PRIVATE ExternalLibraryName)
+
+set(EXTERNAL_PROJECTS_PREFIX ${CMAKE_BINARY_DIR}/external-projects)
+set(EXTERNAL_PROJECTS_INSTALL_PREFIX ${EXTERNAL_PROJECTS_PREFIX}/installed)
+
+include(GNUInstallDirs)
+
+# MUST be called before any add_executable() # https://stackoverflow.com/a/40554704/8766845
+link_directories(${EXTERNAL_PROJECTS_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR})
+include_directories($<BUILD_INTERFACE:${EXTERNAL_PROJECTS_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}>)
+
+include(ExternalProject)
+
+ExternalProject_Add(externalRestcCpp
+    PREFIX "${EXTERNAL_PROJECTS_PREFIX}"
+    GIT_REPOSITORY "https://github.com/jgaa/restc-cpp.git"
+    GIT_TAG "master"
+    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTERNAL_PROJECTS_INSTALL_PREFIX}
+    )

examples/cmake_external_project/main.cpp

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+// We use the example from the main readme file
+
+#include <iostream>
+
+#include <boost/lexical_cast.hpp>
+#include <boost/fusion/adapted.hpp>
+
+#define BOOST_LOG_DYN_LINK 1
+#include <boost/log/trivial.hpp>
+#include <boost/log/utility/setup.hpp>
+
+#include <restc-cpp/restc-cpp.h>
+#include <restc-cpp/RequestBuilder.h>
+
+using namespace std;
+using namespace restc_cpp;
+namespace logging = boost::log;
+
+// C++ structure that match the JSON entries received
+// from http://jsonplaceholder.typicode.com/posts/{id}
+struct Post {
+    int userId = 0;
+    int id = 0;
+    string title;
+    string body;
+};
+
+// Since C++ does not (yet) offer reflection, we need to tell the library how
+// to map json members to a type. We are doing this by declaring the
+// structs/classes with BOOST_FUSION_ADAPT_STRUCT from the boost libraries.
+// This allows us to convert the C++ classes to and from JSON.
+
+BOOST_FUSION_ADAPT_STRUCT(
+    Post,
+    (int, userId)
+    (int, id)
+    (string, title)
+    (string, body)
+)
+
+// The C++ main function - the place where any adventure starts
+int main() {
+    // Set the log-level to a reasonable value
+    logging::core::get()->set_filter
+    (
+        logging::trivial::severity >= logging::trivial::info
+    );
+
+    // Create an instance of the rest client
+    auto rest_client = RestClient::Create();
+
+    // Create and instantiate a Post from data received from the server.
+    Post my_post = rest_client->ProcessWithPromiseT<Post>([&](Context& ctx) {
+        // This is a co-routine, running in a worker-thread
+
+        // Instantiate a Post structure.
+        Post post;
+
+        // Serialize it asynchronously. The asynchronously part does not really matter
+        // here, but it may if you receive huge data structures.
+        SerializeFromJson(post,
+
+            // Construct a request to the server
+            RequestBuilder(ctx)
+                .Get("http://jsonplaceholder.typicode.com/posts/1")
+
+                // Add some headers for good taste
+                .Header("X-Client", "RESTC_CPP")
+                .Header("X-Client-Purpose", "Testing")
+
+                // Send the request
+                .Execute());
+
+        // Return the post instance trough a C++ future<>
+        return post;
+    })
+
+    // Get the Post instance from the future<>, or any C++ exception thrown
+    // within the lambda.
+    .get();
+
+    // Print the result for everyone to see.
+    cout << "Received post# " << my_post.id << ", title: " << my_post.title
+        << endl;
+}

examples/cmake_external_project/readme.md

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+# example with cmake and restc-cpp as an external project
+
+This example shows how to compile a program using restc-cpp from cmake, delegating the compilation of restc-cpp to cmake.
+
+This is probably the simplest way to use the library, but the external project dependencies does add some extra time each time you run make.
+
+Currently this example is only tested under Linux.
+
+```sh
+~$ rm -rf build/
+~$ mkdir build
+~$ cd build/
+~$ cmake ..
+~$ make
+```