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.
Ok, after this first review, I'd like to highlight what I think are the important things we have 
to agree on to facilitate future PRs. Let me know what you think:

1. Each PR has to be focused to a very specific feature or bug fix. 
2. The PR should not introduce regressions. Tests will be run when available and
manual user interface testing has to be done.