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
``` c#
/// <summary>
/// The name of the type [describing|containing information about] a specific tag helper scope.
/// </summary>
/// <remarks>
/// Contains information about in-scope tag helpers, HTML attributes, and the tag helpers' output.
/// </remarks>
```