Add Planck 2013 cosmology

Added a Planck 2013 cosmology object.  Not the default because it isn't yet published (like WMAP9, which still isn't published either).  Note the paper appears on astro-ph tomorrow, so it might make sense to delay a day to add an arXiv reference -- right now it just mentions the Planck Legacy Archive.

Also -- I can't get the sphinx docs to build at all any more, so I haven't been able to look at the minor changes I made for those.

* 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.