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
Actually, I kind of prefer this format to be honest.  I don't see the value of having a constructor that takes everything.  If you feel different let me know, going to remove the "todo" comments and document this guy.