create failing unit test to demonstrate problem identified in issue #594
remove all the byteorder checks from fits.py, and move them into the Image constructor
(#594)
check for little-endian/big-endian mismatches in Image constructor
(#594)
note bug fix in CHANGELOG
(#594)
use nifty byteorder-swapper to simplify image constructor tests
(#594)
Merge pull request #595 from GalSim-developers/#594

#594 - Image bug fix for little/big-endian issues