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
For a situation where you want to do `@foo + @bar` you'd just do `foo + bar` which is plain text.  As of right now the only time an `@`  can appear in the attribuet value is during a `string` bound property.