Bug 830468 - Rotate images in gallery

Fix for [#830468](https://bugzilla.mozilla.org/show_bug.cgi?id=830468). This patch adds a Rotation function in the 'edit' feature of the gallery.

The approach I took is to redraw the image when someone rotates, as this is by far the easiest way to rotate an image. Thereafter I update all the references to the image so that the other effects now use this canvas. Works pretty neat. Tested on B2G desktop and the Unagi device.

As you can see I have terrible design skills, so I'd like someone from UX to give some feedback as well (plus design some nice icon for the clockwise / counterclockwise rotation).