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 the case that we find a `TagHelper` you're correct, it does not reach here.  It's maintained separately so it does not stack overflow by recursively adding to itself :smile: .  Since it's maintained separately and can't be added until it's fully formed it builds a sub-scope to add children to.  I'll rephrase the comment to make it very clear what type of children are added at this line.