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
grammar worse here since the method definitely does not change the _type_.  again, I think you mean `/// The method in <see cref="TagHelpersExecutionContextTypeName"/> used to add tag helper attributes.`