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.
Didn't know about `iter_docs`, thanks for pointing it out. Good to know `Domain.delete` context as well.