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.
In html5, tag closing for some tags is [optional](http://www.w3.org/TR/REC-html40/struct/lists.html#edef-LI).