Issue #2: wrong naming

Major changes:

* renamed `admin.html` to `instructor.html`
* added some tweaks to instructor's UI (for example mode indicator)
* fixed bug with admins removing other peers from a room when quitting

I'm still very unhappy with this events driven design (look for example at the code changing mode indicator), but I will rework it for v0.2.
Student's code prepares their browsers for streaming when they receive `mode_change` event from the server.

So basically all I want to do is to set to single room mode as soon as teacher appears online.