coverflow image stacking/overlap improvement

Please see screenshots. 

Problem: The items in the coverflow were incorrectly stacking (to the right of the selected item) and was an eyesore when overlapping.

Solution: Override the gallery getChildDrawingOrder 
The solution has been tested, see the 'improvement' screenshot (NB the images to the right of the selected item are now correctly stacked)

![coverflow improvment](https://cloud.githubusercontent.com/assets/7987374/4441842/db6afa1e-47d7-11e4-8f7d-0ec1068eabf8.png)
![coverflow before](https://cloud.githubusercontent.com/assets/7987374/4441843/df17a95a-47d7-11e4-81a8-c0100f30e61c.png)