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
naming here derives from `@addtaghelper` naming so it makes sense.  could of course change the keyword to something like `@usetaghelper` (and later `@removetaghelper` might be `@ignoretaghelper`).
/cc @DamianEdwards