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 both the functions as-is or module-level constants as [here](https://github.com/gammapy/gammapy/blob/master/gammapy/utils/const.py#L28) are OK solutions.

Both have advantages and disadvantages, e.g. module-level constants are less code, but it's harder to make them appear correctly and nicely in the docs ... last time I tried I didn't manage to make them appear at all in the docs (see https://github.com/gammapy/gammapy/issues/111). I'm sure this is possible, but can you attach an example like @larrybradley wrote in the function docstring.

So bottom-line: I don't care either way, but we should merge this soon ... there's much bigger fish to fry this week for the 1.0 deadline.