Example of plotting coordinates on an all-sky map (final pull request)

This should be the final pull request as discussed in issue #2726. There were some problems with my repo, so I had to re-create my branch and so github doesn't allow me to update the original pull request.
I think it might be a little clearer to instead put the multiplication by ``u.degree`` here instead of as part of the call to `SkyCoord`.  Then you can say something like: "... for DEC between -90 and +90 degrees (dec_random).  Finally, we multiply these values by degrees to get an `~astropy.units.Quantity` with units of degrees."