disable unassigned warnings for fields in the PreserveAttribute class

Resolves issue #76 whereby visual studio would spew warnings
that RefitInternalGenerated.PreserveAttribute.X is never
assigned.
Merge pull request #77 from ghuntley/issue76

disable unassigned warnings for fields in the PreserveAttribute class
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)