Add functions for age and comoving_volume to cosmology

This adds functions for `age` and `comoving_volume` corresponding to the methods of the same name. This follows discussion on the astropy list (the thread starting with this message: http://mail.scipy.org/pipermail/astropy/2013-December/002938.html) where some people mentioned they had difficulty finding this functionality because there weren't function versions.

Pinging @aconley & @eteq who might like to take a look.

I plan to open separate PRs to add an inverse function and comoving volume element, which were other changes mentioned in the same thread.