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
what in Razor will use this property?  either Razor handles running the individual helpers at the right times (avoiding buffering and so on) or it calls a helper manager at multiple points in the generated file (and the manager handles the content behaviour).  since you've said offline that you're going with the second approach, this property isn't useful to Razor.
actually we want as much as possible done w/in Razor.  we don't need a helper manager to do the Right Thing(tm) w.r.t. `ContentBehavior` inside Razor.  so I'm all for limiting or removing the helper manager concept and keeping this property.  the calls up into MVC (or whatever layer is above Razor) will just need to wrap calls to an individual tag helper, avoiding any requirement for Razor to understand types such as `TagBuilder`.