Add sorting, make buddylist look more like mocks

* Added sorting with headers: https://github.com/uProxy/uproxy/issues/469
* Modified uProxy buddylist styling to look more like mocks
 * Non-uProxy friends are no longer clickable
 * Changed background from grey to white
 * Offline contacts now appear grayscale
* Modified isOnline logic: users are now considered offline if we know about their instances, but all of those instances are offline (even if they are online with another app)

You can see a screenshot of the new buddylist here: https://drive.google.com/a/google.com/file/d/0B6oXFcuW01xTbDBjTXI1RGNtNGM/view?usp=sharing