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 think we should hardcode `norm(0.75) = 0.67448975019608182 `
That has two advantages:
- We don't need scipy as dependency.
- It run much faster (it's kind of a waste of overhead to import scipy and get the norm object, just to calculate a fixed number. Instead, there can be a comment line in the code that explains where the 0.6744... comes from.