Add HEALPIX reprojection

Tool to convert HEALPIX images to Galactic or ICRS coordinates, based tightly on @cdeil's function in gammapy.  See https://github.com/healpy/healpy/issues/129.
Yes, we should use this function here, but it hasn't been tested with WCS objects that were created using HEALPIX FITS files as input:
https://github.com/astropy/astropy/blob/master/astropy/wcs/tests/test_utils.py

My guess would be that it doesn't always work properly and we definitely need to add tests for files such as the ones at
http://lambda.gsfc.nasa.gov/toolbox/tb_pixelcoords.cfm either there or here.