Add mad_std, fwhm_to_sigma, and sigma_to_fwhm functions

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 
How about a modul level constant
`gaussian_fwhm_to_sigma = 2.35...`. Defining a function just to multiply with a constant seems quite a high level of effort.
And again, I suggest hardcoding `(2.0 * np.sqrt(2.0 * np.log(2.0))) = 2.35...`.