Use new style string formatting for cosmology instances.

* Use "{...}".format instead of % for all __repr__
* Homogonize the syntax of the __repr__ in a few places -- Flat instances
  no longer print Ode0, since that isn't free.
* Add unit tests for __repr__.  I'm not sure if this is a good idea or not,
  however.
* closes #1701
Formatting fix.

* Change {:4g} to {:.4g} for Tcmb, although it doesn't seem to change
  the results.
String formatting fix for python 2.6

* In test_cosmology, use {0:s} instead of {:s} in new-style string
  formatting for py 2.6 compatability.
Make name truly optional instead of setting to __class__.__name__

* Modify all __init__s to default to name = None if not provided
* Modify all __repr__ to only output the name if it is set.
* Modify tests to match
Documentation update for new __repr__

* Add some documentation for name parameter, with example.
  Also use that to introduce alternative dark energy models.
* Fix incorrect documentation for m_nu units.
Merge pull request #1705 from aconley/cosmo_newstring

Use new style string formatting in cosmology instances