Feature/activity stream

this adds the activity stream

to be precise it does the following:
 * save votes to the event log (server side)
 * add a new tab to the event view
 * bucketize eventlog messages to clean up the log (see event-activity-stream.js if you're interested in details)
   * this means that it will not show every single vote, it instead sums them together
 * the activity log gets the messages as follows:
   * all songs from the eventlog rest api at first
   * all votes from the eventlog rest api at first
   * all subsequent songs from the websockets

some caveats:
 * our client side WS-library websocket-client.js currently only supports binding a single callback to an event. the callback for "song_played" is currently registered to so-event-song.js. I just added a notifyActivityStream(..) function to forward this event to the activity log. if we do the same for the votes, we are golden. I did this because it's easier than rewriting and retesting the ws-client (and it works just as well for our usecase of **two** different events [song_played and song_voted]).

as for the websockets for votes:
since @lfichtinger needs to implement this for the votes, I'll integrate it once she is done; feel free to merge this PR

note: if you want me to write proper tests for the bucketization it's probably the last thing I'll do in this project until I've reached my 150 hours xD

here a screenshot of the log:
![stream1](https://cloud.githubusercontent.com/assets/275897/5555505/621ea6f4-8cb2-11e4-82e5-499c69882ecf.PNG)