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.
(For context, the Domain.delete method appears to have been written by an intern two years ago before we had peer review; while it looks like it works, it's certainly not a well-worn/well-tested/often-used piece of code.)