Merge pull request #1144 from blink1073/master

Clear the blit background when the image changes
Update find_contours docstring to use r/c notation

This update fixes #1140.

Other docstrings will probably need to be updated as well.
Merge pull request #1145 from jni/find-contours-docstring

Update find_contours docstring to use r/c notation
Fix swirl docstring to use (row, column) nomenclature