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
pass the element name to the manager when creating a helper.  current approach requires an implicit "between End and Start" state and that will soon be unmanageable.  as-is may also be difficult to reuse helpers on a per-element basis.