Add cosmology methods

This PR makes all the cosmology convenience functions also available as methods of the cosmology object.

I was originally opposed to doing this, but after using the package for a while, I've changed my mind: it seems much more convenient to import a single cosmology object (say WMAP7) and then have all the functionality attached to that instead of having to import several objects or functions.

There is also a minor doc change that removes a sentence that no longer applies.