Support Jcrop.setImage

Hi jonasundderwolf.

Great piece of code you have there, bravo and many thanks for your work.
Perhaps you'd be interested in my fork? Changelog: 
- move jcrop variable to global scope -- allows access to Jcrop api;
- add Jcrop.options.boxWidth and Jcrop.options.boxHeight support through model field -- lets us define a maximum allowed space for the cropping widget when source image is changed through Jcrop.setImage;
- IMAGE_CROPPING_JQUERY_URL = None -- prevents inclusion second jQuery copy when, for example, it's used in frontend and jQuery has already been loaded.

Updated doc to reflect these.

Again, thank you for your work, you're great.

Cheers,
Peter