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
- remove mention of `<see cref="TagHelpersExecutionContextTypeName"/>` since this will break documentation XML file (reference can't be resolved in this assembly).
- either "a tag helper" or "tag helpers"