Imagemagick should be optional

Alright, this is fairly small change, but possibly with bigger impact. I may be missing some other use cases that could be broken by this.

This change is made for the workflow when there are pure developers on the team and they don't want to install *garbage* like Imagemagick to their machines. However they still need to be able to generate valid *.css files with attributes for sprites. That means only people working with graphics will generate sprites + json and commit to repository.