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
- remove noise words "that is" (8 other occurrences in this PR)
- clarify this summary a bit more.  for one thing, the current wording begs the question "so, who finds these helpers?"  mention something about the provider being long-lived, having application scope (well, will have application scope), and its responsibility to discover and describe tag helpers referenced from Razor directives.