Merge pull request #4641 from jreback/arthops

ENH: (GH4629) Support for using a DatetimeIndex/PeriodsIndex directly in a datelike calculation
TST: test for GH4633, bool(obj) behavior

BUG: GH4633, rever to GH1073, whereby __nonzero__ always raises for all NDFrame objects
DOC: docs to add detail on gotchas for true value testing