Added dV_comoving function and tests.

Following discussion here: 
http://mail.scipy.org/pipermail/astropy/2013-December/002965.html
and @nhmc mentioned it might be useful to have this function included. 

The dV_comoving function allows for an easy method of integrating
over a comoving volume. It is useful when, for example,
integrating a function that depends on another function whose
sensitivity changes with redshift.

I've also included tests and a method in the funcs library as well.