Start android application from watchface

Goal: start the gps from the watchface even if the android app is not launched (or was killed by the system to save memory).
Update PebbleSDK to version 1.12
Imperial units for altitude fields
Manage units changes in MainActivity (instead in GPSService)

* could refresh fields (units&data) in watchface immediatly when units changes
Bugfix with Toast
v1.2.0-beta2