Add TagHelperDescriptorResolver implementation.

- This involved also adding a TagHelperTypeResolver and TagHelperDescriptorFactory.
- The TagHelperTypeResolver is responsible for determining the format of lookup text's used throughout the tag helper system.  By default it handles the following formats:
"assemblyName"
"specificType, assemblyName"
- It also restricts what types are considered TagHelpers.  In this implementation we only accept public, non-nested, non-abstract, non-generic TagHelpers.
- The TagHelperDescriptorFactory is responsible for converting a Type to a TagHelperDescriptor.
- Added tests to validate TagHelperDescriptorResolver, TagHelperTypeResolver and TagHelperDescriptorFactory.

#99
#158
nit: I was suggesting leaving the `TagHelperDescriptorComparer` constructor as `private` and copying its comparisons into `CompleteTagHelperDescriptorComparer`.  but that wasn't a big deal.