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
the "block.codegenerator" ref doesn't seem to fit.  should this perhaps be
``` c#
/// The <see cref="Block"/> responsible for this <see cref="TagHelperCodeGenerator"/>.
```
or
``` c#
/// The <see cref="Block"/> that this <see cref="TagHelperCodeGenerator"/> is responsible for.
```