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.
This is one of the things I'd say are not important right now. Both the instructor and the user is going to know in what mode they are based on their interfaces. Instructors will see rooms and students will see a long or short list of users depending in what mode they are on. *Yes* we want to add that later on.