Add ol.DeviceOrientation

Retrieve the [orientation of the device](http://www.w3.org/TR/orientation-event/). All the angles are in radian.

demo: http://dev.camptocamp.com/files/fredj/deviceorientation/examples/device-orientation.html

tested with Chrome mobile and firefox on Android.

TODO:
- [x] tracking property (activate/deactivate)
- [x] compass heading (see `event.absolute` and `webkitCompassHeading`)
- [x] better demo (something like: http://ariya.github.io/js/marblebox/)