Add example datasets

We should add some example datasets (i.e. images or PSFs e.g. as FITS files) to `photutils` that can be used for the docs. They could also be used for unit tests.

Let's use this wiki page to gather some proposed datasets: https://github.com/astropy/photutils/wiki/Example-Datasets

One tricky question is where to store these files ... for the larger images we don't want them in the `photutils` repo, but we do want to have them available for the docs build. Maybe a separate `photutils-examples` repo or upload them on `data.astropy.org`? Or we rely on external data services and use e.g. Astroquery to download them?

What I've done in Gammapy is add a [gammapy.datasets](https://gammapy.readthedocs.org/en/latest/datasets/index.html) sub-package similar to `skimage.data` or `sklearn.datasets` that are then easily available for the docs, the test or the end user ... maybe we could do the same for `phoutils`?

@astrofrog @eteq @bsipocz @mwcraig What do you think?
@larrybradley I agree with all your comments about `GaussianPSF`, but  please let's address them in #48  ... some changes would be simple to make (like changing parameter names), but for elongated Gaussians I don't know what the analytical integral formulas are and in general I think some parts of the code should be mored to `astropy.modeling` but this will probably need some more discussion.