Improve Cropping Behavior #166 by Sethdenner 

This is an upgrades the Original feature in #166 with the current development branch

> sorl-thumbnail has been a great asset in our Django based web project however the implementation of cropping left a little to be desired (or at least the implementation didn't make a lot of sense to me) so I made some modifications.
> I modified the cropping behavior in the following ways:
> The crop parameter now takes x,y coordinates of the top left corner as well as width/height of the crop region.
> Cropping occurs before scalling.
>Feel free to merge this into the main project if you also feel that this is an improvement. If you have any additional notes I will be happy to make further modifications if this isn't exactly what you had in mind.

https://github.com/mariocesar/sorl-thumbnail/pull/166