lookback_distance for astropy.cosmology.core

Here is my pull request, following the input I received.
It calculates a lookback_distance within the FLRW Class.

Again, I think that term (physical distance) is loaded, and can't really be justified.  Physical distance is a pretty ambiguous quantity in cosmology.  If you want to relate it to something physical, perhaps 'light travel distance'.  

Plus, there's a typo here.