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
also
- what point is there in a `TagHelperExpression` that isn't used to generate property values
- refer to `TagHelperExpression` using `<see .../>` (if you absolutely insist that class must exist)