add project_id to get_events API response

In order to facilitate things like adding assets from a related project to the session map, we need to have a way of traversing to project_id from session_id.

Here's the proposed JSON response with the new info at the bottom:

```
{
    "events": [ 
       {
            "data": null, 
            "event_id": 64082, 
            "event_type": "heartbeat", 
            "latitude": null, 
            "longitude": null, 
            "server_time": "2013-07-17 14:16:06", 
            "session_id": 4912, 
            "tags": null
        }, 
        {
            "data": null, 
            "event_id": 64083, 
            "event_type": "cleanup_session", 
            "latitude": null, 
            "longitude": null, 
            "server_time": "2013-07-17 14:19:30", 
            "session_id": 4912, 
            "tags": null
        }
    ], 
    "number_of_events": 2,
    "project_id": 12
}
```