Minor cleanup of `astropy.vo` docs
Add instance name to built-in cosmo docstring, output class name with __repr__

* For the built-in cosmologies (like WMAP9), add information to
  the docstring stating they are instances of built in classes,
  and identifying the class.
* Change __repr__ to print the type of class first, then the name
  as an argument (e.g., instead of WMAP9(...), FlatLambdaCDM(name="WMAP9", ...)
* closes 1544
Add _ones_like to quantity_helper, to allow q**0
Ensure that ones_like is used for numpy 1.6; also add _arg
Rewrite additions so it will pass if private functions disappear
Merge pull request #1658 from cdeil/vo_docs

Check of `astropy.vo` docs examples for 0.3 release
Merge pull request #1683 from aconley/cosmo_docstring

Add instance name to built-in cosmo docstring, output class name in __repr__
Merge pull request #1696 from mhvk/quantity-helper-ones-like

BUG: _ones_like not in quantity_helper, hence  q**0 fails