REF: rename module mixed_linear_model, add to api
DOC: add mixedlm to documentation, separate rst
Merge pull request #1826 from josef-pkt/lme_api_docs

REF/DOC Lme api docs
phreg initial commit of working code
Added summary; other fixes; ready for PR
Added R results
Minor cleanup of phreg.py
Changed test script to address python 3 issue with exe (exec no longer used)
Renamed time to endog for clarity
Add missing data option to signature
Remove local test code
Need to explicitly convert extra arguments to ndarrays
Add fields to summary
Update compat import of ordereddict
Fix issues with 2.6 (set comprehension) and 3.3 (test import)
More 2.6, 3+ compatability fixes
Fix import for python 3 compatability
Add HR's to table, change CI to apply to HR, not coeffs
A few vectorizations to improve performance
Minor fix to test script
Adjusted setup.py to give access to sandbox.results
Added score_obs; various cleanups.