DOC: complete stats.boxcox docstring.  Add add boxcox functions to refguide.

Also do a PEP8 cleanup of `boxcox`.
TST: add tests fox stats.boxcox.
DOC: complete stats.boxcox_llf docstring.  Also PEP8ify that function.
TST: add tests for stats.boxcox_llf.  Also add some input parameter handling.

Add also a reference to the original paper of Box and Cox.