Numpy 1.5 doesn't have count_nonzero

* In FLRW.__init__ np.count_nonzero was used to count the number
  of non-zero neutrino masses.  This doesn't work under numpy 1.4 or 1.5,
  which are still supported by astropy.  Use a try/else against
  AttributeError to use count_nonzero if available, len(nonzero()[0])
  otherwise.
Documentation update for neutrino masses, other small fixes.