Bluetooth branch

This branch adds the following functionality:
* bluetooth relay server: one device acts as a hub, and obtain a mavlink data connection (via bluetooth, usb, etc...). If 'enable bluetooth relay server' is checked in the preferences, the hub will start listening for, and accepting incoming connection request from interested bluetooth devices (i.e: google glass). On successful connection, these devices will be able to receive, and send data through the mavlink connection via the hub bluetooth connection.
* bluetooth device selection screen: this is added to be able to select 'bluetooth relay server hub' to connect to, on top of the regular telemetry serial bluetooth module.
* checkbox in the user preferences to enable or disable the bluetooth relay server.
* updates to the BluetoothConnection class to support connection to 'relay server hub'.