Alarms

```python
>>> # create an alarm with default properties
>>> alarm = Alarm(my_device)  # my_device is the SoCo instance on which the alarm will be played
>>> print alarm.volume
20
>>> print get_alarms()
set([])
>>> # save the alarm to the Sonos system
>>> alarm.save()
>>> print get_alarms()
set([<Alarm id:88@15:26:15 at 0x107abb090>])
>>> # update the alarm
>>> alarm.recurrence = "ONCE"
>>> # Save it again for the change to take effect
>>> alarm.save()
>>> # Remove it
>>> alarm.remove()
>>> print get_alarms()
set([])
```
I'd recommend to define a the valid play-modes centrally (I think they're used in ``soco.core`` as well) and reuse them where necessary.