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.
Looks like `np.logical_not` is not available in `numpy 1.6` making travis-ci fail:
https://travis-ci.org/astropy/photutils/jobs/28003579#L589