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
Will put `[NotNull]` just on this overload.  Since all other overloads call into it 1 `[NotNull]` functionally works.  As for an engineering guidelines standpoint I brought it up with @davidfowl because we should probably be consistent.