Tag helpers parse tree rewriter tests

- Added tests to validate TagHelperParseTreeVisitor when dealing with valid HTML elements.
- Added understanding to the ParserTestBase for TagHelperBlock's
- Added a helper class MarkupTagHelperBlock to make building test TagHelpers easier.
- Fixed some existing tests that "new"d up the RazorParser and didn't obide by the new contract (to provide optimizers).

The implementation portion of this is in a separate PR here: https://github.com/aspnet/Razor/pull/109


Issue: #71
looks like you use each theory data set once.  would be much easier to understand these tests if the data set were defined immediately above the test method that uses it.