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.
We don't want to pollute the global name space. Notice how I try to put all related logic in its own 
name space.

I'd add those in the variable definition within the newPP function in `js/events.js`