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
this is not the correct place to throw an `ArgumentNullException`.  nothing should be assigning `null` to the `TagHelperRenderer` property.  so throw the `Exception` in that setter.