create multiple low credit notification levels

Currently there is only one notification level. Add a list in the config of all the notification percentage levels that should be checked, and send the relevant notifications when the levels are reached.
However, mixing data and schema migrations like this is probably a bad idea. The correct approach is described in http://south.readthedocs.org/en/latest/tutorial/part3.html (using the ``orm`` object).

I'm kind of in favour of just throwing the old values away though.