BUG: TimeGrouper sortedness / API fix (GH4161,GH3881)
BUG: Bug in TimeGrouper/resample when presented with a non-monotonic DatetimeIndex would return invalid results,
BUG: Bug in index name propogation in TimeGrouper/resample
closes #4161

API: TimeGrouper has a more compatible API to the rest of the groupers (e.g. `groups` was missing) 
closes #3881
