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
Nope, the - 2 is to account for the end part of the start AND end part of the tag AKA:
```
<  div   >   // or
<  div  />
```
The parser groups the end ```>``` with the ```/``` if it's there.