Add RotatingFileHandler.__del__ that closes stream

Eliminates 5 of the `ResourceWarning` warnings mentioned in GH-391.

### Without:

```
$ PYTHONWARNINGS="d" .tox/py33/bin/nosetests -q 2>&1 | grep -c ResourceWarning
13
```

### With:

```
$ PYTHONWARNINGS="d" .tox/py33/bin/nosetests -q 2>&1 | grep -c ResourceWarning
8
```

Cc: @mcdonc, @mnaberez