ENH: stats: Rewrote stats.obrientransform:

* Vectorized and simplified the computation.  For largeish arrays,
  this gives up to 60 times speed improvement.
* Copy-edited the docstring, and added 'References' and 'Examples'.
* `TINY` changed from 1e-10 to `np.sqrt(np.finfo(float).eps)`.
* Added another test.