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 
Since the name is already long, how about `gaussian_fwhm_to_sigma` and `gaussian_sigma_to_fwhm`?