REF: Use fixed params in test. Closes #910.
ENH: add transformation and fit for constraints
TST: add preliminary test for Poisson.fit_constrained
DOC: add try_fit_constrained example file (experimental for OLS and Poisson)
ENH/REF add fit_regularized to Poisson, returns results, with tests for params, bse
TST: add test, tvalues, pvalues, more (2 warnings for no Stata llnull)
ENH: add start_params option
TST fix py3, add start_params to one exampl
BUG: adjust df_model, df_resid for k_constraints
TST: add test results for cov_type robust, not implemented yet
TST: fix incorrect comparison df_resid in test,
     attach constraint to results
TST  add test for fit_constrained function with GLM
ENH add GLM.fit_constrained, almost all copy-paste not DRY
ENH GLM add llnull, fix starting mu to use predict (incl. offset, exposure)
TST/REF: return constraint result from fit_constrained function
         tests for GLM.fit_constrained and llnull
BUG: GLMResults not all cached attributes are registered for remove_data
BUG: add family to _init_keys, remove from unit tests
TST: add tests for GLM.fit_constraint Logit case
TST: add tests for predict, mu
DOC docstrings for fit_constrained