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.
Logging the stacktrace of the error is more useful than just saying that there was an error. `unicode(e)` usually doesn't have much information in it. For example:

```python
>>> unicode(KeyError('pear'))
u"'pear'"
```

I'd either not do a blanket try/catch and let the error bleed through, or add a logging.exception statement before raising.