Rename period->timespan to be clearer; make timespan=None behave itself.
Rename period->timespan to be clearer; make timespan=None behave itself.

..and also so that I don't have to do the dirty pass-a-global-as-a-local-because-python-gets-pissy-about-assigning-to-globals-in-a-local-scope thing

kousu/isi,kousu/isi