Introduce AppVeyor CI for Windows

The included configuration aims to compile and test builds of
sqlite3-ruby gem against a native Windows environment provided by
AppVeyor platform.

Add `devkit` task that prepends both compile and native ones and
ensures RubyInstaller's DevKit is enabled in the PATH prior
compilation.

appveyor.yml

@@ -0,0 +1,26 @@
+---
+version: "{build}"
+branches:
+  only:
+    - master
+    - 1-3-stable
+clone_depth: 10
+install:
+  - SET PATH=C:\Ruby%ruby_version%\bin;%PATH%
+  - ruby --version
+  - gem --version
+  - gem install bundler --quiet --no-ri --no-rdoc
+  - bundler --version
+  - bundle install
+build_script:
+  - rake native gem
+test_script:
+  - rake test
+artifacts:
+  - path: pkg\*.gem
+
+environment:
+  matrix:
+    - ruby_version: "193"
+    - ruby_version: "200"
+    - ruby_version: "200-x64"

tasks/native.rake

@@ -9,6 +9,20 @@ BINARY_VERSION = "3.7.17"
 URL_VERSION    = "3071700"
 URL_PATH       = "/2013"
 
+# prepend DevKit into compilation phase
+if RUBY_PLATFORM =~ /mingw/
+  task :compile => [:devkit]
+  task :native  => [:devkit]
+end
+
+task :devkit do
+  begin
+    require "devkit"
+  rescue LoadError => e
+    abort "Failed to activate RubyInstaller's DevKit required for compilation."
+  end
+end
+
 # build sqlite3_native C extension
 RUBY_EXTENSION = Rake::ExtensionTask.new('sqlite3_native', HOE.spec) do |ext|
   # where to locate the extension