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.
Agreed. Except that I think having current mode set in a variable in a numeric format is a good idea.