Add @addtaghelper directive. 

- Also added some infrastructure pieces for it such as the ITagHelperDescriptorResolver and the default implementation TagHelperDescriptorResolver which will be filled out in a later commit.
- Reworked some extensibility points to allow accessibility of the descriptor resolvers per offline discussions.
- Fixed existing tests to work with new RazorParser.
- Validated directive syntax tree creation, errors and code generation.

#111
I'd prefer the test code by default replace the `TagHelperDescriptorResolver`.  shouldn't need to include `@addtaghelper` to _unit_ test other parts of the system.  we'll also see just how unclean the `TagHelperDescriptorResolver` replacement is.