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.
And I'd modify it to include something like

```python
for doc in related_docs:
    doc = doc['doc']
    assert doc['domain'] == self.name
    db.delete_doc(doc)
```

out of a health sense of paranoia