Add NominalGEE and OrdinalGEE to api

This PR adds NominalGEE and OrdinalGEE to the API.  Some related enhancements are bundled in too, specifically:

* Privatize the Multinomial and MultinomialLogit classes, make _Multinomial the default family for NominalGEE.  Automate the setting of the number of levels.

* Make binomial the default family for OrdinalGEE.

* Docstring changes related to above.
GlobalAddsRatio is in the `sm.cov_struct` namespace.