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 
@larrybradley If you leave those as standalone functions, how about renaming to the more explicit `gauss_fwhm_to_sigma` and `gauss_sigma_to_fwhm`?