MNT: Documented API change

DOC: Finished documentation to API change on img_subtraction

MNT: Another attempt to remove tests from coverage report

MNT: Another attempt to get coveralls to ignore the test/ and testing/ folders

MNT: Removed .coverage and moved .coveragerc to .coverage

MNT: Another test for coveralls to ignore tests/ and testing/

MNT: Another test to ignore tests/ and testing/

DOC: Removed io from coverage