Add an entropy-based crop

We could inspire http://drupal.org/project/smartcrop, however the approach is quite different:
1/ Smartcrop reduces step by step either side, until it arrives to the required size, we must continue as if the required size is zero or is a single point (so it might happen that that point does not fit multiple scale).
2/ Smartcrop usually changes the layout (by zooming in), while Image Focus just change the starting position when scale and crop.