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
- CLR or HTML attributes?  or do you mean properties of the helper's class?
- why mention both "metadata" and "communicate"?  isn't this just a class that "describes" something?
- nit: anything but "functionality"