Add mask option to exposure.equalize_hist with test.

Remove print and fix comparision to None

Update test to make it clear there is a difference

Fix mask logic and `equalize_hist` in `__all__`.

Fix handling of mask and tweak mask test

Another run at travis

Update travis to avoid conflict
Merge pull request #1109 from blink1073/add-exposure-mask

Add mask option to exposure.equalize_hist with test.