Add mad_std() and constants to convert Gaussian FWHM <-> sigma

This PR adds a standard deviation estimator using MAD, and two simple functions to convert between Gaussian FWHM and sigma.  These calculations were often repeated in `photutils`.

CC: @cdeil, @astrofrog 
To make superclear what this is, i think I would phrase this as:
```
Factor with which to multiply Gaussian 1-sigma standard deviation(s) to convert them to
full width at half maximum.
"""
(and the same for the below).

p.s.  A commit with these changes could include `[skip ci]` since it does not affect travis. With that, ready to merge... Thanks for your patience!