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
I might be off-base but why rebuild the `Span`?  doesn't look like anything will change in the loop above.  then, could break off the loop as soon as you find a value for the `name` variable.  and this line would be
``` c#
return new KeyValuePair<string, SyntaxTreeNode>(name, span);
```