Create TagHelper specific C# code Generation.

- Added TagHelperChunk generation.
- Added CSharp visitors to understand TagHelperChunks and render corresponding C# code.
- Refactored some code in the CSharpCodeVisitor so it could be utilized in other classes.
- Added a CSharpFieldDeclarationVisitor to render declaration pieces for TagHelper's
- Added metadata to represent specific TagHelper code generation constructs.

#72
Add code generation tests for tag helpers.

#72
Add extensibility point for TH attribute code gen.

- Enabled the CSharpCodeBuilder to create the CSharpCodeVisitor which exposes a CSharpTagHelperCodeRenderer which is indirectly used to render attribute values.
- Fixed up some existing classes that just new'd up the CSharpCodeVisitor class to instead take in the decorated version.

#119
Add test for TagHelperAttributeValueCodeRenderer extensibility.

- Tested that we can successfully plugin our own TagHelperAttributeValueCodeRenderer.
- Moved some existing TagHelper test logic into its own base type to enhance code reusability.

#119
Add @addtaghelper directive.

- Also added some infrastructure pieces for it such as the ITagHelperDescriptorResolver and the default implementation TagHelperDescriptorResolver which will be filled out in a later commit.
- Reworked some extensibility points to allow accessibility of the descriptor resolvers per offline discussions.

#111
Add tests to validate @addtaghelper directive.

- Fixed existing tests to work with new RazorParser.
- Validated directive syntax tree creation, errors and code generation.

#111
Addressed code review comments.
Addressed code review comments.