Use astropy.cosmology instead of built-in cosmology when possible.

* Import astropy.cosmology when possible and use instead of
  built-in cosmology class.
* Add deprecation warning to built in cosmology
* astropy 0.3.0 or more recent required because the astropy.cosmology
  objects return things with units.  This -is- tested for;
  if the version is less than 0.3.0, the built in cosmology
  class is used instead.
* datasets/generate.generate_mu_z no longer supports optional
  **kwargs passed to cosmology object; this would be complicated
  to implement when using astropy.cosmology because it takes some
  positional (rather than named) arguments.  However, this option
  wasn't used in the figures anyways.
* Affected figures in the book tested to be the same in python 2.7:
   4.10, 8.2 8.4, 8.5, 8.11
* Nosetests pass.