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 math still doesn't work out.  need more than _three_ children for the `attributeChildren` to be non-empty a few lines below.
probably best to remove this early escape and just return if `attributeChildren` ends up empty.  looks like neither `Skip()` nor `Take()` will throw even if the `Children` collection starts out completely empty.