Add `screenshot_url` field to /api/v1/user/schedule

This just uses the screenshots we took for Facebook thumbnails. It also happens that these would be convenient to use for our Android app.

Though, we'll probably want to make changes to the screenshots taking work better for the Android app:

- [ ] take screenshots of all non-empty weeks
- [ ] get rid of the next/prev week controls
- [ ] change aspect ratio (make image less fat)

`http://localhost:5000/api/v1/user/schedule`:

```json
{
  "screenshot_url": "http://localhost:5000/static/schedules/98f3f7c43ecdce4469ec3251137c6992.png", 
  "schedule": [
    {
      "end_date": 1362507600000, 
      "prof_id": "oleg_michailovich", 
      "course_id": "ece402", 
      "id": "50f51edf8aedf4e11ba0da11", 
      "building": "M3", 
      "room": "1006", 
      "term_id": "2013_01", 
      "section_type": "SEM", 
      "section_num": "003", 
      "class_num": "4479", 
      "start_date": 1362504600000
    }
  ]
}
```

`http://localhost:5000/static/schedules/98f3f7c43ecdce4469ec3251137c6992.png`:

![image](https://f.cloud.github.com/assets/159687/2338361/2ad502e4-a4a8-11e3-8566-b7be7b957fe3.png)

cc @jasperfung