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
however
- I'm suggesting code generation could fairly easily work around #129 (which reads like a parsing issue) by rendering _all_ the children directly to the output
- while #129 isn't implemented (if it's required), should not be silently ignoring the children. silent failures are bad.