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
strange.  why resolve potentially the same `lookupText` differently the second time `Resolve()` is called?  if behaviour change is necessary for some test, make it explicit and add a settable `bool` property that is read in this method.