generate diff images and save in output directory

[GelLiNN]

@d3ming

Wraps imagemagick compare tool in tagcompare's image.py, and invokes that from tagtester's main.py

[d3ming]

Some general things:

1. Make sure tests / flake8 pass by running tox locally
2. Add test for the function you added in image.py

[coveralls]

Coverage increased (+0.03%) to 94.822% when pulling f8f0988 on GelLiNN:master into 7ccc605 on paperg:master.

[d3ming]

I don't think you need this. I think this flag is just set for cases where the test needs to use the flag. I'm honestly not sure I thought that through when I first used it so you shouldn't use it if you don't need to :)

[d3ming]

Looks good overall, just a couple of minor comments. Good to merge once you fix that
👍

[coveralls]

Coverage increased (+0.03%) to 94.822% when pulling a62de36 on GelLiNN:master into 7ccc605 on paperg:master.

[coveralls]

Coverage increased (+0.03%) to 94.822% when pulling 9f44a5d on GelLiNN:master into 7ccc605 on paperg:master.