Create TagHelper specific C# code Generation.

- Added TagHelperChunk generation.
- Added CSharp visitors to understand TagHelperChunk's and render corresponding C# code.
- Refactored some code in the CSharpCodeVisitor so it could be utilized in other classes.

#72 

I split up the test portion of this PR to reduce noise.  PR: https://github.com/aspnet/Razor/pull/135
sorry, `<see cref="TagHelpersExecutionContextTypeName"/>` works (misread it) but mention is very confusing.  I think you mean `/// The name of the method in <see cref="TagHelpersExecutionContextTypeName"/> used ...`