Add simple diagnostic output to InterfaceStubGenerator.

Diagnostic messages use Visual Studio format (see:
http://blogs.msdn.com/b/msbuild/archive/2006/11/03/msbuild-visual-studio-aware-error-messages-and-message-formats.aspx)
Remove unnecessary complexity and fix code style
Remove unnecessary this
Add warnings for method overload in refit interfaces
Extract linq statements from foreach
Add new file to all tests projects
Use extension method
Merge pull request #79 from nekresh/generator-diagnostic

Add simple diagnostic output to InterfaceStubGenerator.