Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
The factor is needed to use the MAD as a standard deviation estimator (see the "Relation to standard deviation" section of http://en.wikipedia.org/wiki/Median_absolute_deviation).  It's exact value is:

```
>>> from scipy stats import norm
>>> factor = 1. / norm.ppf(0.75)
>>> print(factor)
1.48260221851
```

I will add a `mad_std()` function to `astropy.stats` that simply returns `mad(data) / norm.ppf(0.75)`.  Sound good?