Add a "delete_domain" and "delete_user" management command

Addresses this ticket: http://manage.dimagi.com/default.asp?130184.

`delete_domain` deletes the domain couch document and all couch documents with `doc.domain` set to the given domain. `delete_user` deletes the user from postgres and deletes the couch document.
Simon said something similar. I interpreted the exchange on the fogbugz ticket as a request for total deletion from the user though.