Full width video in chat window

Related [Trello card](https://trello.com/c/NRxtnwdD/213-as-a-user-during-a-video-call-i-see-three-lines-of-text-scrollback-and-the-rest-of-the-conversation-window-is-filled-by-the-vide).

Well, I didn't expect this one to become that big. 

I've added a new `devel/chat-devel.html` HTML file to ease debugging the CSS for the chat window. Samples:

### Video call

![capture decran 2013-08-30 a 14 41 59](https://f.cloud.github.com/assets/41547/1057357/b7b0a738-1171-11e3-8e68-84577c449e24.png)

### Simple text chat conversation

![capture decran 2013-08-30 a 14 42 16](https://f.cloud.github.com/assets/41547/1057358/bcd31dd6-1171-11e3-8913-93b289089038.png)

Buttons for debugging call offer & establishment screens are also provided.

Large video call window screen (the one on the right) is not done yet because not part of the current Trello card; will be addressed in a next PR when working on [this other one](https://trello.com/c/AlJNrB3U/215-as-a-user-during-a-video-call-my-buddy-and-my-video-scale-intuitively-based-upon-how-i-resize-the-conversation-window).