Add catalog DNS name to Quilt+ URIs

[drernie]

Right now, the URIs only contain information about the package.
It is often desirable for users (and programs) to infer the actual URL where a person can view a package.
This requires passing that information via some other mechanism, which is often difficult and sometimes impossible.

[drernie]

To do this right, we should also update the documentation to formally define Quilt+ URIs.