BUG: Bug in TimeGrouper/resample when presented with a non-monotonic DatetimeIndex would return invalid results. (GH4161)
BUG: Bug in index name propogation in TimeGrouper/resample (GH4161)
API: TimeGrouper has a more compatible API to the rest of the groupers (e.g. ``groups`` was missing) (GH3881)
Merge pull request #6350 from jreback/groupby_sorted

BUG: TimeGrouper sortedness / API fix (GH4161,GH3881)
BUG: properly tokenize & and | and exprs with strings