Replaced half-hearted ROI code with crop code.

I really wasn't feeling the ROI code and didn't see it getting any less
crufty in the future, so I removed it and implemented a crop method instead.

Also cleaned up the code a bit and added a lot of docstrings.
Merge branch 'master' of https://github.com/wil-langford/FishFace

Merging code written at the office with code written at home.