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
What i'm trying to say is: bufferable attributes means that they "could" be buffered.  But if we find that there's no C# code inside of the value then we treat it as a plain text value, meaning no need to buffer.  Will clarify comment.