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
this doesn't handle `@foo + @bar` or any other non-string attribute value the parser will split into multiple chunks.  in other words, I now agree w/ your comment on not reducing the nesting but you need to handle `!isPlainTextValue` here.