Add TagHelper parse tree rewriter.

- 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 

**NOTE:** This does not support malformed tag helpers.  It expects all tags (that are tag helpers) are validly formed.  That fix will come with: https://github.com/aspnet/Razor/issues/104

*Also* I opened a separate test PR due to size here: #110

Issue: #71
- in minimal cases such as `<p>` and `<p/>`, `Children` will contain 3 tags.  every other case has more children -- attributes and whitespace
- so the math here is suspect.  note as well the next block will take no elements if `tagBlockChildcount==2`.  this should be `tagBlockChildcount<=2`