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 
p.s.  Here, I think it really is not needed at all, but in general try to remember to use `np.asanyarray` so that subtypes get preserved.