Improve UX when cropping image size is less than needed

[arogachev]

Instead of applying styles (red border around the crop frame as in current implementation), we can just use minSizeoption of Jcrop (which is now hardcoded to [5, 5]). That way user can not initialize crop frame or resize it to size smaller than minSize. With warning approach user needs to repeat resizing multiple times to figure out what the correct size is because warning state is changed only after releasing mouse.

Related: #106.

[anrie]

As mentioned in #106 it would be helpful if you would demonstrate your approach in a PR.

[MRigal]

ping @arogachev