Add management command to dump all MAC's

Resolves #714. This has never taken more than 4 minutes to run, so it should be safe to use in practice.

Current output
```
$ head maint_mac_db 
000000000001 zone.scf jcasctest More_than_90_days
000000000002 zone.ext-morrow ODFW-PIX More_than_90_days
```

Output of the management command:
```
$ head cyder_mac_db 
000000000001 zone.mu Mike-AndersonCo More_than_90_days
000000000002 zone.engr lennon More_than_90_days
```

You can test the command by running `./manage.py mac_dump -f cyder_mac_db`. If you don't pass the `-f` flag, the results will just be printed on stdout.

I still need to test if this script's output is compatible with the output of `radius-report`.