Add Pebble notifications

Added the ability for Telegram to send notifications to a connected Pebble smartwatch (using the [reference documentation](https://developer.getpebble.com/1/03_API_Reference/03_Android/) with slight modifications).

This can be toggled in Notification settings and defaults to "Off".

I have been testing this over the past few days with my Pebble connected to my Nexus 5/Nexus 7 (2012).