ENH: Always do true division on Python 2.X

Note: this version actually forces `a / b` to always do truediv as well.
It's less complicated to maintain if we do it that way, but it's also
possible to have this `a / b` use div and `a.div(b)` use truediv, just
might be strange. This is definitely different from numpy behavior.

We also may need to add the division future import to more places.
DOC: Add notes about truedivision
Merge pull request #5439 from jtratner/make-div-do-truediv

ENH: Always do true division on Python 2.X
API: raise SettingWithCopy when chained assignment is detected