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 
:+1: to `np.asanyarray()`--I generally like any functions that can work on arrays to be able to accept anything `asanyarray` will accept, in analogy with almost all of Numpy's built-in functions.