Refactor to expose Users and Role

We did a fair amount of work to simplify what is needed for external realms to integrate with Nexus.
This work was done for 1.2, but the API has changed a bit for 1.3 (current trunk)

The Crowd Realm should be updated to take advantage of this. This will allow plugins to have access to users email, for notifications. It will also allow Users to be listed in the User page, as well as be mapped directly to Nexus roles.

Take a look at:
https://docs.sonatype.com/display/NX/Nexus+Security+FAQ

And the example:
https://docs.sonatype.com/display/NX/Security+Example