Add support for push notifications on class seat openings

Go to "schedule" tab of a course:

![image](https://cloud.githubusercontent.com/assets/159687/2634541/5ff5054a-be7b-11e3-9641-77b5d36ff5dc.png)

Tap the "add alarm" button to add an alarm for a course:

![image](https://cloud.githubusercontent.com/assets/159687/2634838/6237dbd6-be7e-11e3-899a-f27a9ea0176c.png)

Corresponding server-side changes: https://github.com/UWFlow/rmc/pull/140