Better to use filenames which are commonly used in the python word then
custom ones.

Using timezone.now() will return naive datetimes if USE_TZ is not
enabled, which is what we want.
Resolves errors of comparing naive datetimes with aware datetimes.

This should always be used instead of Django's silent version.

Offers by default are sorted using Meta class ordering. However,
`get_offers` was doing two queries and combining them together.
Effecitvely bypass the ordering.

Test for ordering basket offers was written in the process not being
aware of standard ordering. I left it there as it can be useful test in
future (when somebody removes standard ordering).

Fixes #964