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
thanks for the reference.  that section includes the following text and something similar about single-quoted attribute values.  Note points 2 and 4.

----
A double-quoted attribute value is specified by providing the following parts in exactly the following order:
1.an attribute name 
2.zero or more space characters 
3.a single "=" character
4.zero or more space characters 
5.a single """ character
...