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
- isn't this the base class for CLR properties in the tag helper which aren't simple values?  current summary doesn't imply that at all
- "attribute's" -> "attributes" (no apostrophe)
- remove one or the other "expression"