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
my complaint was about the _name_ because "PropertyVisitor" implies the class visits properties rather than generates property declarations when it hits a particular chunk.  but I don't think this class is needed so the point is moot.