Bundle imageutils for photutils 0.1 release

As discussed in https://github.com/astropy/imageutils/issues/15, for the photutils 0.1 release we should bundle `imageutils` in `photutils/extern`, because it will not be available in the Astropy core as `astropy.image` yet.

Technically this can be done by copying over files into `photutils/extern` or maybe by creating a git submodule in `photutils/extern` ... to be investigated / discussed here.

cc @larrybradley @astrofrog