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
`DefaultTagHelperExecutionContextVariableName` should only be used to initialize a property in the `TagHelperContext`.  if this and `DefaultTagHelperScopeManagerVariableName` aren't intended to be overridden, you shouldn't have renamed them to include "Default"