Fix nested tags in script tag.

#115
Test nested tags in script tag fix.

#115
Add TagHelper 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
Validate Tag Helper registration system functionality.

- This involved adding tests to cover GetTagHelpers methods on the default tag helper descriptor provider.

#70