L.A.Cosmic: cosmic-ray detection and cleaning

This PR adds the L.A.Cosmic (http://www.astro.yale.edu/dokkum/lacosmic/) algorithm used to identify and clean cosmic rays from single images.
I thought they were interchangeable.  I didn't know that `~` was preferred or than `logical_not` isn't in `numpy 1.6`.  I've fixed this now, thanks!