Fix for iOS Safari not rendering some stage background colors

PIXI.Stage.prototype.setBackgroundColor converts color numbers to hex strings without padding (e.g., 0x00ff00 becomes "#ff00"). Safari iOS does not handle colors specified this way correctly:

![0x00ff00](https://f.cloud.github.com/assets/249571/651835/33fa1674-d484-11e2-94bc-aa0b9dfdfd91.png)

This fix pads the representation with leading "0" characters (e.g., 0x00ff00 becomes "#00ff00").