App styles

Updated chat window and sidebar styles to better match Zurb's mockups.

I don't know what the sidebar header will become so I'm leaving it untouched for now.

![capture decran 2013-06-10 a 17 32 22](https://f.cloud.github.com/assets/41547/632419/130fd4ce-d1e3-11e2-86e8-e58ad0bb1627.png)

Caveats:

- I've implemented very basic call rejection, a.k.a `window.close()`; this is to be implemented more conveniently in a next PR
- for the online status pills, I'm just adding a css class to the element depending on what is stored in the `user.presence` value, which is always "disconnected" for now; this should be fixed in another PR
- in order to display the caller's avatar, we'd need to access the `otherUser` as a `User` model instance, though it's not the case atm; to be fixed in a next PR

I suggest @Standard8 to review this one.