Improve Cropping Behavior

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:

1. The crop parameter now takes x,y coordinates of the top left corner  as well as width/height of the crop region.

2. 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.