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
Ya this is a fail/overlook on my part (some of my initial "hack" code).  Arrays/IEnumerables aren't handled via TagHelpers so in the case of IEnumerables I need to throw.  When I add support for Dictionaries https://github.com/aspnet/Razor/issues/89 i'll revisit this piece.