DISC: Period.__le__
There are a bunch of issues outstanding that relate to `Period` ops and comparison.  AFAICT making a decision about the comparison issue will snowball into resolving (some of) the ops issues.

#5202 ENH: Period ops NaT & timedelta ops
#10798 Date / Datetime in Period Index
#6779 Adding Period and Offset not implemented
#13077 ENH/API: Decide what to return Period - Period subtraction
#17112 MultiIndex - Comparison with Mixed Frequencies (and other FUBAR)

Right now two Period objects are comparable iff they equal `freq` attributes.  AFAICT this is to avoid guessing in cases where the "correct" answer is ambiguous.  But there are some other cases with an obviously correct answer.  Namely, if the `per1.end_time < per2.start_time`, then it should be the case that `per1 < per2` unambiguously.  This intuition also extends to `datetime` and `Timestamp` objects that do not lie between `per.start_time` and `per.end_time`. 

For cases with overlap there are a couple of reasonable approaches.  My preferred approach is lexicographic: first compare `per1.start_time` with `per2.start_time`.  If they are equal, then compare the `end_time`s.  Then we treat `datetime` and `Timestamp` objects as analogous to zero-duration periods.

Thoughts?