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's "on the generated Razor page"?  if you meant the method, note the method may be `static` in the base class, this property could be given the value `Html.StartWritingScope()`, ...  just leave that clause out.