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
That's a fair point. 

We could break the interface apart into two.  A registration system and a provider system.

Suggestion: ```ITagHelperProvider``` for GetTagHelpers and ```ITagHelperContext``` for the reg/unreg methods?  Whatcha think.