Add a message handler.

[fouksf]

No description provided.

[bgrozev]

I'm not sure I'm looking at a finished version of this (the current commit id is fbe8bcd). It looks ok to me, except for the 'bridgeIsDown' signalling. Maybe this is due to the change from one format to another, but currently: the code reading the presence looks for 'available' and it shouldn't, the addVideoBridgeStatus function should probably be renamed since (now) it accepts a boolean, the "bridgeIsDown" variable seems unused.

There seem to be quite a few error messages which are the same, or nearly the same, and don't provide any details about the error to the user. Emil wrote elsewhere that we should show them nevertheless, because they show to the user the fact that something went wrong. I suggest a generic 'oops' function in MessageHandler to be used for all these. This way we can more easily control if a dialog is shown, or if another form of notification is used.