Alternative resolutions

Optionally create a n-sized thumbnail for every thumbnail sorl-thumbnail creates.
This very helpful for "retina"-like HDPI screens.

Credits go to: @relekang and @philipn
Fix bugs related to alternative-resolutions and cropping

If the crop parameter is set with pixels we need to scale them according
to the resolution

Cherry-picked fc427368 by @relekang
Merge pull request #219 from coagulant/retina

Alternative resolutions