Merge pull request statsmodels#4633 from josef-pkt/backport_0.9.0

Backport 0.9.0

Merge pull request statsmodels#4571 from josef-pkt/MAINT_setup_readme

MAINT: update setup.py and README.rst

Merge pull request statsmodels#3458 from bashtage/add-module-tags-to-…

…docs

DOC: Add module tags to docs

Merge pull request statsmodels#3064 from TomAugspurger/ros-sort

COMPAT: pandas.sort in ros.py

Version 0.6.1 Release

Version 0.6.0 Release.

Version 0.6.0 Release Candidate 2

Version 0.6.0 Release Candidate 1

Version 0.5.0

Version 0.5.0 Release Candidate 1