rsplit on different timezone format

in models.py line 105 it's trying to do an rsplit on the "+" character but in my particular python + django install the timestamp has a different format.

This is the line:

```
timestamp, tz = resp["SubmittedAt"].rsplit("+", 1) 
```

And this is the timestamp:

```
u'SubmittedAt': u'2011-09-23T07:39:47.9044908-04:00',
```

i.e. there is no + sign in that timestamp
