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
- sorry I missed one: remove "Arbitrary"
- nit: verb is "look up" (two words) according to [Merriam-Webster](http://www.merriam-webster.com/dictionary/lookup) and [Dictionary.com](http://dictionary.reference.com/browse/lookup)