Add base CourseAlert models
Make CourseAlert an ABC; add methods to send and delete
Add tests and fixtures for BaseCourseAlert
Add GcmCourseAlert subclass to send out Android push notifications
Send push notifications automatically on each sections update
Add POST /api/v1/alerts/course/gcm endpoint and tests
Add DELETE /alerts/course/gcm/<string:alert_id> endpoint and test
Oops, fix bug with BaseCourseAlert.delete_expired

... deleting items with expiry dates in the future instead of in the past. Also
fix terrible test that did not catch this case. Double oops! Thanks for the
catch @mduan!!
Merge pull request #140 from UWFlow/david-push-notifications

Add support for push notifications to our Android app