Add mask option to exposure.equalize_hist with test.

Adds an optional `mask` argument to `exposure.equalize_hist`.  If given, the equalization is calculated based on the unmasked region, and applied to the whole image.  A test is included.  Addresses #1107.
Actually, this can be confusing, as we saw earlier. How about, "equalize the image based only on the pixels where `mask` is `True`."