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, this class gives no value.  The tag helpers cannot be inside of the `Execute` method body because that restricts the ability for us to use tag helpers in `@helper`s.

Move all these variables to private fields on the generated class.