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 
I don't feel strongly about this either, but I thought "array-like" included lists.  I'm happy to take out the `np.array()` too.