Allow unregistering names if the package is not yet used

[Facundo Batista]

It happens sometimes that the user registers a name (charm, bundle, etc.) and then finds it was an error. It may be that the type of package was different, or the name was misspelled, etc.

It would be great if the Store allows a name to be "unregistered" if the package is still not "used" (e.g.: no revisions were pushed for it, but there may be other conditions).

Thanks!