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
There can be a plainTextValue when an object is not bufferable.  This would break functionality.  I'd have to do 

```
if (bufferableAttribute && isPlainTextValue)
{
}
else if (bufferableAttribute)
...
```