Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
For this I get a puzzling `RuntimeWarning`:
```
/Users/deil/Library/Python/3.4/lib/python/site-packages/astropy-1.0.dev10714-py3.4-macosx-10.10-x86_64.egg/astropy/stats/funcs.py:157: RuntimeWarning: invalid value encountered in greater
  filtered_data.mask |= do * do > varfunc(filtered_data) * sig ** 2
```
Is there a way to avoid this (worth making an issue or PR for `astropy.stats` or catching it in `photutils`)?