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?
I get an import error at this step --

```python
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-65b26022d1bc> in <module>()
----> 1 from photutils import datasets

/Users/mcraig/Development/astronomy/astropy-org/photutils/photutils/__init__.py in <module>()
     13 from .aperture import *
     14 from .psf import *
---> 15 from .detection.core import *
     16 from .detection.findstars import *
     17

/Users/mcraig/Development/astronomy/astropy-org/photutils/photutils/detection/core.py in <module>()
      3                         unicode_literals)
      4 import numpy as np
----> 5 from ..utils.scale_img import img_stats
      6
      7 __all__ = ['detect_sources', 'find_peaks']

/Users/mcraig/Development/astronomy/astropy-org/photutils/photutils/utils/__init__.py in <module>()
      7 try:
      8     # not guaranteed to be available at setup time
----> 9     from .sampling import *
     10 except ImportError:
     11     if not _ASTROPY_SETUP_:

ImportError: No module named sampling
```