Allow registering of default Hibernate UserTypes

It would be really useful if it was possible (typically in a plugin) to register default Hibernate UserTypes for commonly used types so that they do not have to be specified in the mapping closure of each domain class. Any explicit mapping that is present should, of course, take precedence.

This would enable, for example, the Joda-Time plugin to register PersistentDateTime as the UserType for DateTime, etc. which can then be used without explicit mapping in projects using the plugin.