aheckmann/gm — pull request #238 by longlho (2013-11-27 23:14:54+00:00)
Title: support compare for ImageMagick, #230

Couple of notes:

Response from ImageMagick compare is a bit different:
- 0 is 2 images are the same
- 1 is 2 images are dissimilar
- 2 is actual error

Therefore, I put some conditions around differences between GM & IM. Happy to revise if there're issues.