Split screen editing demo

Usage instructions:

1. Check out this PR or branch
2. Start the server as per normal: ``node webodf/httpserver.js``
3. Point a browser towards http://127.0.0.1:8124/webodf/programs/editor/src-splitscreen.html
4. Log in on both clients (no user validation is performed)

# TODO
* [x] Add support for OT
* [x] Pause/play support to interrupt flow between clients
* [x] Quicker login
* [x] Randomise cursor colours to make it easier to see which user is which
* [x] ~~Fix op priorities so it's per memberid~~ (not an existing problem. My understanding here was flawed)
* [x] ~~Let Bella loose in these!~~ (won't happen in this PR)

# Pics
![screenshot from 2014-06-17 18 12 07](https://cloud.githubusercontent.com/assets/1052079/3297694/2b1369a8-f5f7-11e3-8be1-98c4d09247f5.png)

I agree. I've added a second callback queue called ``successfulSyncCallbacks`` which is only fired when a successful sync is performed. The first has been renamed to ``syncAttemptCompleteCallbacks`` make it clearer that it's called on every sync attempt. So far, this is only used internally.