Clarify the QCompass coordinate system

The compass behaves like the other sensors: it is
measured relative to the top of the device.

Change-Id: Iddc4ad6a16f8b09d26a9d300155324bb8d378b76
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>
Compass sensor implementation for Android

Change-Id: Ic13d2d1e400a92b2f0c3c83e279c58b799f9d06b
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>