Add more info to manually created scoring analytics event.

@wedaly, @stephensanchez, @mulby, @rocha

When a submission is finally scored (gone through the entire workflow), we emit an event that looks like:

```python
{
    'time': datetime.datetime(2014, 3, 30, 20, 48, 45, 339065),
    'event_source': 'server',
    'event': {
        'points_possible': 20,
        'points_earned': 8,
        'submission_uuid': u'1284eaee-b84c-11e3-b4e1-040ccee02800'
    },
    'context': {
        'course_id': u'edX/Enchantment_101/April_1'
    },
    'event_type': 'openassessment.workflow.score'
}
```

This was discussed in https://github.com/edx/edx-platform/pull/3118