Add a system whereby collections of units can be enabled after the fact.
Add additional units in CDS/Vizier format
Move % and --- to the cds.py unit collection
Make Crab completely CDS-specific
Fixups after rebase
Fix doc build by using an approach that will get unicode attributes on modules correctly on Python 2.x
Add a list of distutils commands that don't require downloading dependencies.
Add neutrino mass support.
Add neutrino mass support.
Add unit testing (comparison with mathematica), fix some bugs that revealed.

* Use Mathematica to numerically evaluate the exact expression
  for neutrino mass density, use that to set up some unit tests.
* Fix some minor issues uncovered by those tests, like _nu_y having
  units.
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.
Add neutrino mass information to __repr__ for cosmology objects.

* Adjust __repr__ for FLRW and all subclasses to output neutrino information.
  For consistency, since %r is used for m_nu (a quantity), also use %r
  for Tcmb0 and H0.
* closes #1302
Correct documentation for return type of absorption distance, make numarray

* This showed up during the rebasing of this branch: the documentation
  for absorption distance was incorrect, saying it returned a quantity.
  Fix the docs, and make the return a np.array rather than a list
  for consistency with all other functions.
* Affects the unit test for absorption distance.
Fix documentation typos pointed out by @nhmc
Further documentation tweaks.
Require m_nu to be a Quantity following @eteq recommendation.
Fix units on m_nu in default cosmologies.
Fix default arguments to have units for m_nu, also fix cosmology tests.
Use assert_allclose rather than assert_almost_equal (depracated).

* assert_almost_equal seems to not work correctly in some cases.
  This is showing up in test_distances_scipy, which claims that
  two distances which agree to 12 decimals don't agree to 7 decimals.
  Since assert_almost_equal is deprecated for not always being
  a good comparison, replace with assert_allclose.