Addressed more 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.
Addressed code review comments.
Addressed code review comments
Reverted code.
Addressed code review comments
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.