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

This pull request is for a rather small bug fix to address an issue David found when using the form of the Image constructor that takes a NumPy array directly, where it makes an image with garbage in it if the byte order of the input NumPy array is not the native one.

Previously, two routines in fits.py had checked for this problem when reading in FITS images from file.  Those checks have been removed, and now the only place where this is checked is in the Image constructor (which therefore works whether the NumPy array is passed in directly, or was read from file by the routines in fits.py).  The form of the check and fix is also a bit simpler.

The code change here is quite small, and I have a hard time imagining how what I did could break anything... but since we've had some subtle issues after changes in image.py for people with different systems / versions of NumPy / whatever, it would be good to have a few people to check that tests still pass for them.  Tests pass on both of my systems (Mac and Linux cluster).

@dkirkby , please check it out and see if everything works for you on this branch.