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 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.