Removed unneeded namespaces.
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.