Some minor fixes for stats.wishart addition.

- fixes to docstrings (references, see also, added in 0.16.0)
- use ``np.linalg.slogdet`` where appropriate
- group tests for multivariate distributions
I use `make html` in the `doc` directory.  The numpydoc extension in the numpy source code (i.e. `numpy/doc/sphinxext`) is in my python path. Sphinx version is 1.2.3.