new deleted_consumers resource

I needed a way to grab all of the deleted consumers, so I added a new resource
under deleted_consumers. You can call it like so:

/candlepin/deleted_consumers - returns all deleted consumers
/candlepin/deleted_consumers?date=<date> - returns all deleted consumers created since the date specified

This is only available to super admins; normal users and org admins shouldn't
ever have to use it.