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 
In this and the above example, is the `print()` statement necessary considering that the return values are just numbers?  This might just be me but it always seems to be just unnecessary noise when it isn't directly testing how some object is displayed by `print()`.  I've tried to remove it from doctests where it isn't necessary.  Not insisting though.