replace deprecated sky2pix/pix2sky with world2pix/pix2world
raise an exception if insantiating FITSFigure from WCS with latest
astropy [still fails tests!]
fix all WCS-based tests
added note about WCS deprecation to CHANGES
fix bugs!  getheader only works on FITS files
Merge pull request #183 from keflavich/wcs_sky2world

Replace deprecated sky2pix/pix2sky with world2pix/pix2world