Fixed PNG transparency when cropped or resized

* We can now crop and resize PNG and keep transparency
* I've also added a configurable URL to SLIR to customize SLIR entry point in a CMS.

Example : `SLIR/w200-c200x320/SLIR/Test/images/camera-logo.png`

![camera-logo](https://cloud.githubusercontent.com/assets/380026/2617320/fcc105e4-bc10-11e3-8eea-890ccf0f636a.png)