Make resize_to_fill match the behavior derived from that API call in the other CarrierWave processors

Also, I tweaked #resize_to_fit, which was off by a pixel on several examples. Refactored maciej's spec (#1)  to parallel the way they are done in the carrierwave gem. (You will see that his commits are included here.) Added additional specs for #resize_to_fit and #resize_to_fill.

If you merge, you might want to bump the version, because of the change in behavior of #resize_to_fill

If the old #resize_to_fill behavior is desired, I suggest that be moved into a different method.