Add photutils.utils functions

I added everything that was in `photutils.utils` (re: https://github.com/astropy/imageutils/issues/9)

Should `fix_prf_nan` stay in `photutils.utils`?  It seems more specific to `photutils`. 

The travis-ci setup-egg info builds are failing because of this:
https://travis-ci.org/astropy/imageutils/jobs/31270906

You have to put all imports into the `if not _ASTROPY_SETUP_:` block.