CMake support

[FlorianRhiem]

This PR adds a CMakeLists.txt to the project:

• The list file and possibly installed config files provide a library oatpp::oatpp.
• If BUILD_SHARED_LIBS is set, a shared library will be built. Otherwise (the default) a static library will be built.
• If OATPP_INSTALL is set (the default), an install target is generated that will install the library, its headers and cmake config files to the CMAKE_INSTALL_PREFIX.
• If OATPP_BUILD_TESTS is set (the default), the tests are built into a single executable and a test target is generated that will run these tests using CTest. To allow this, test/AllTestsMain.cpp was added that will provide a main() for running all tests if the macro OATPP_ENABLE_ALL_TESTS_MAIN is defined during compilation.
• To maintain CMakeLists.txt, new source and header files need to be added to the list of files for the oatpp library, or the oatppAllTests executable.

Fixes #7.

[lganzzzo]

That is a great PR!
Thank you for your help and interest in the project!