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 some changes to make these 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)
- [ ]      \(not related to screenshots, but) query params to this API endpoint to specify week

`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