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 firefoxon Android.

TODO:
- [ ] tracking property (activate/deactivate)
- [ ] [device motion] (http://www.w3.org/TR/orientation-event/#devicemotion)
- [ ] compass heading (see `event.absolute` and `webkitCompassHeading`)
- [ ] better demo (somthing like: http://ariya.github.io/js/marblebox/)