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
I really like this approach but where is it enforced that both `Foo` properties have the same (or similar-enough) types?  fine to say "nowhere yet" and file a follow-up issue.  but let's not lose track of this.