Store cropping parameters

[ezuhaib]

Currently the cropping parameters are passed on to ImageMagick which returns the cropped image without persisting those parameters. If we save these cropping parameters, we may be able to:

1. Add an "adjust thumbnail" feature, same as in facebook.
2. Facilitate modifying of croppings by loading the crop box in the previous cropping.

If such a feature is implemented, unlink this repo from the original and publish as a seperate gem.