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
realize this is a longstanding issue but it's very confusing that `WriteMethodInvocation()` doesn't both start and and the invocation!  please file a bug to rename those two methods to `WriteStartMethodInvocation()` and (perhaps) add _real_ `WriteMethodInvocation()` methods.