Add Planck 2013 cosmology

* Add Planck2013 cosmology object.  Not the default because
  not yet published.  More Planck results are not expected until
  2014, so hopefully the name is safe.
* Mention Planck2013 object in documentation.
* Add very minor unit test for Planck2013.
Update Planck13 reference, correct omega_8 value.
Merge pull request #895 from aconley/planck13

Add Planck 2013 cosmology