Tag helpers registration system

- This registration system accepts tag descriptors that it then uses to feed requests for tag helpers.
- Added some infrastructure pieces that are used to build up valid tag helper descriptors.

#70
If you unregister a catchall descriptor it gets replaced:
https://github.com/aspnet/Razor/pull/97/files#diff-8d2ff3aa54ffb798f305022beb1300daR61