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
really?  can't come up with a shorter unique name?  for example, each `TagHelper` only has a single `ContentBehavior` and we shouldn't need a different variable when a wildcard helper is applied to different tag names.