Merge pull request #3613 from jreback/groupby_dtype

BUG: Fix incorrect dtype on groupby with ``as_index=False`` (GH3610_)
BUG: Add reduce_if_possible keyword to ``groupby`` to allow reduction from

    DataFrame -> Series if groups are unique. Regression from 0.10.1,
    partial revert on (GH2893_) with (GH3596_)

CLN: renamed reduce_if_possible -> squeeze

DOC: added v0.11.1 example