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
just check `typeof(string) == attributeDescriptor.PropertyInfo.PropertyType`.  `string` is `sealed` and `IsAssignableFrom()` doesn't return `true` if an implicit conversion exists.