Pebble Smartwatch support

My new pebble smartwatch finally arrived here (yay!). So first thing to do, obviously: Add support for it to Xabber.

With the attached code, the pebble now shows incoming messages on it's display.

Of course there is a setting to disable the function in the settings menu if you don't want to use it. Settings translations in english and german included.

Works just fine for me.