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
Absolutely shippable without that requirement.  Any case you want to enable more than just "string"'s v1 you will just use a ```TagHelperRazorExpression``` problem solved.  As for getting rid  of the manager, not going to happen.  No way i'm going to generate all of the logic involved in rendering a tag helper.  That would require knowledge of MvcTagHelper's, TagBuilder's, ViewContext's and more within Razor core.