Implement ordering on MinKey / MaxKey

For any task that involves inspecting chunk ranges with the Python driver, it would be helpful if MinKey and MaxKey would compare always less than / greater than any other value (this is currently not the case, only __eq__ and __ne__ are implemented).

Implementation suggestion:

MinKey.__le__ = lambda self, other: True

MinKey.__lt__ = lambda self, other: other != MinKey()

MinKey.__gt__ = lambda self, other: False

MinKey.__ge__ = lambda self, other: other == MinKey()