Fixes #82 by simplifying the port resolution in the Domain class.

This commit just fixes #82 by simplifying the port resolution in
the domain class. That affects every place where the URL is
displayed, and solves the problem of when the user copies URLs
with either port 0 or 80, as described in the ticket.

*	modified:   grails-app/domain/docker/registry/web/Registry.groovy
- Just simplifying the port resolution.