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
@NTaylorMullen that separation will help but it doesn't quite address the later part of my comment.  Is `ITagHelperRegistrationContext` (maybe `ITagHelperRegistrar` is better?) an interface that _must_ be implemented or a facet of our default `ITagHelperProvider` implementation.  Put another way, what are the scenarios for providing alternate implementations of these interfaces?