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
Not sure what you mean by a valid name.  Does that fall under the malformed tags category?  And ```FirstHtmlSymbolAs()``` iterates through the symbols and returns the first that meets the provided ```HtmlSymbolType```, in this case ```HtmlSymbolType.Text``` so in this case it's really just skipping over the whitespace prior to an html attribute (whitespace is only thing allowed).