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
if (all?) attributes are automatically `MarkupBlock`s, why do we need all the code in `ParseSpan()`?  your comment implies `ParseSpan()` should only ever see stuff like whitespace before, between, and after attributes.  if whitespace doesn't land in either a `Block` or a `Span`, why is `ParseSpan()` needed at all?