Some enhancements to the interface.

* new CSS classes for video sources (local/remote/teacher)
* new CSS classes for video sizes (original/medium/small)
* original=640×480, medium=320×240, small=160×120
* countdown message now really visible
New signalling WIP.  Changing JS code to use JS patterns
Global state: keeping track of users should work fine

As long as WAMP reconnects properly…
Broadcasting: works for constant list of peers.  Needs further enhanc.
Calling in when someones broadcasting works. Yay!

Also: I incremented minor version number in `package.json`, because
apparently I forgot to do that a few weeks ago.
Instructors can't stop some else's broadcast.
Transforming local stream to be a mirror, not real image from webcam

(easy CSS! :) )
WIP: small group discussions.  Backend for that works.

Now I need to change student's front-end behavior.
Groups discussions: calling someone doesn't work.
Ask for webcam access at the beginning of the session
Fixes #16 (thumbs/large video stream), next up: countdown

When I implement countdown, I'll be at feature parity with previous
version. Except that I'll have less bugs. :-)

A few things to take care of:
* someone wants to join the small groups call later on
* someone drops out of small group call
* all instructors drop out
* I think that's all…
We can now handle new students joining during small groups discussions

Next:
* handle someone dropping out of the calls in SMALL_GROUP
* handle instructors dropping out (in BROADCAST is OK, but not in
  SMALL_GROUP)
* bring COUNTDOWN to life (maybe let the server do actual countdown)
Students dropping out from group discussions - 50% done

What's missing: calling in upon join to the new group. Maybe send it by
the server? (`server_pubsub.js:111`)
Removed "test page" because UI is quite advanced right now.
Counting down: works. Some bugs to fix: exist.
Pretty important change: switched to "call me on demand" model
Fixed some issues when new people join in during group discussions
Added TURN server settings
Beep! Beep! When we countdown is soon, do informal beep
Added: corner case of instructors dropping during a SMALL_GROUPS call

WIP: ping functionality. Huh, strange.