[Design] Add TagHelperDescriptorResolver implementation.

- Added a basic TagHelperDescriptor creation mechanism to the TagHelperDescriptorResolver.
- Added an ITagHelperTypeResolver to be responsible for determining valid TagHelper types.

#99
The type resolver is pretty darn large and I definitely feel like it should live as its own being.... This `TagHelperDescriptorResolver` should only be concerned with going from `Type` => `TagHelperDescriptor`.  Otherwise it'd be responsible for `string` => `Type`, Valid `TagHelper` resolution AND everything it already does.  Would be far too much.  

As for the interface it's there because `Microsoft.AspNet.Razor` does not contain the concrete type.  The runtime does.