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
- "TagHelper" doesn't add much to these property names.  "TagHelper" is already part of the class name and "Runner", "ScopeManager" and so on are already unambiguous.
- I have no concerns about the _values" here :smiley: