BUG: Bug in groupby with first/last where dtypes could change (GH3041_)
DOC: docstring updates in core/common.py for _possibily_cast_to_timedelta
Merge pull request #3044 from jreback/groupby_3016

BUG: Bug in groupby with first/last where dtypes could change (GH3041_)
TST: str.contains respects na parameter, GH2806
BUG: str.contains ignored na argument GH2806
Merge pull request #3048 from y-p/GH2806

BUG: str.contains ignored na argument GH2806
ENH: value_counts() can now compute relative frequencies.
DOC: update RELEASE.txt