develop get_session API call

[hburgund]

takes session_id as parameter and returns all info about that particular session_id.

• events
• assets/envelopes
• votes
• listening history

Each of the above should be in a top-level node of the JSON return object.

This will be useful for generating session maps and other data displays for particular sessions.

[hburgund]

At this point, this should clearly be a new api/2/ request, perhaps:

GET api/2/sessions/:id/

though this would suggest simply returning the basic info on the session, not all the additional data.

Maybe:

GET api/2/sessions/:id/history/

??