Move time operations for Series into class _TimeOp

* clarify/simplify time arithmetic-specific code into a separate class
* cleanup extraneous `set` and containment checks in branches
* refactor validation checks to separate function
* combine branches and dry out repetitive code
* move convert_to_array out of arith method
* self --> left; other --> right [possibly clearer for future refacotr]
* change handling of DateTimeIndex (no longer need additional return)
* add np1.6 check for integer --> timedelta conversion under the hood
ENH drop_level argument for xs
Merge pull request #4180 from hayd/xs_drop_selected_level

ENH: drop_level argument for xs
Merge pull request #4613 from jtratner/refactor-datetimes

CLN: Refactor series datetime arithmetic
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