ENH: Add unit keyword to Timestamp and to_datetime to enable passing of

    integers or floats that are in an epoch unit of s, ms, us, ns
    (e.g. unix timestamps or epoch s, with fracional seconds allowed)
    (GH 3540)
TST: disallow slicing a timeseries with floats

TST: manage truediv in py3 for unit comparisons
BUG: make sure that nan/none like values to Timestamp are returned as NaT
Merge pull request #3818 from jreback/epoch

ENH: Add unit keyword to Timestamp and to_datetime