Addressed code review comments.
Add TagHelper parse tree visitor.

- The visitor looks for TagBlock's that match registered TagHelpers and rebuilds them into TagHelperBlock's.
- Added the code generator and corresponding chunk for a TagHelperBlock.
- Added syntax tree specific objects & helper methods to create accurate tag helper structures.

#71
Reverted code generation specific pieces.
Addressed code review comments
Rebased and changed visitor name.
Renamed file.
Fix test to work with TH rewriters.

- Added understanding to the ParserTestBase for TagHelperBlock's
- Added a helper class MarkupTagHelperBlock to make building test TagHelpers easier.
- Fixed some existing tests that "new"d up the RazorParser and didn't obide by the new contract (to provide optimizers).

#71
Add tests to verify TagHelperParseTreeVisitor.

- Added tests to validate TagHelperParseTreeVisitor when dealing with valid HTML elements.

#71
Addressed code review comments.
Renamed tag helper visitor to rewriter in tests.

- Updated namespaces.
Refactored CodeGeneratorContext to CodeBuilderContext.

- Needed to separate the context's of "generation" and "building" to enable the communication of the TagHelperProvider.