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
that's a later stage which doesn't have much in the way of diagnostics right now.  besides, there's plenty of degenerate cases where the file will compile but not do what the user requested due to the ignored chunks.