Add imageutils as a dependency for photutils

I suggest we make https://github.com/astropy/imageutils a required dependency for photutils.
This will allow us to move the image utils functions we have in [photutils/utils](https://github.com/astropy/photutils/tree/master/photutils/utils) to `imageutils` and simply import it in `photutils` without having to do the [optional import dance](http://astropy.readthedocs.org/en/latest/development/testguide.html#tests-requiring-optional-dependencies).

The plan is to propose `imageutils` to be moved into the Astropy core in a month or two, so I don't see any issue with temporarily depending on this package.

@astrofrog @eteq @larrybradley OK?