ENH: Improve error message for PeriodIndex to infer_freq. Closes #5841.
BUG: Bug in selection with missing values via .ix from a duplicate indexed DataFrame failing (GH5835)
Merge pull request #5847 from jseabold/fix-5841

ENH: Improve error message for PeriodIndex to infer_freq. Closes #5841.
Merge pull request #5849 from jreback/dup_selection

BUG: Bug in selection with missing values via .ix from a duplicate indexed DataFrame failing
BUG: fix issue of boolean comparison on empty DataFrames (GH5808)
Merge pull request #5810 from jreback/bool_ops

BUG: fix issue of boolean comparison on empty DataFrames (GH5808)
Fix bug 5846