Add instance name to built-in cosmo docstring, output class name in __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", ...)
* Can certainly be saved for 0.3.1 or later.
* Small change to sphinx documentation noting that built-in cosmologies are classes,
  not functions (see #1544)
* closes #1544