Scales:
- Fix #chunks when used with std_chunk=True (there was one too many at
the beginning)
- Scale.end gives the end of the last chunk (instead of its start)
So scale.begin -> scale.end gives the full range of the scale.
`kfet_day` now returns an aware datetime.
ScaleMixin:
- new method `get_by_chunks` which use only one query and ranks
elements according to the scale. Elements are returned by a generator
for each scale chunk (and all chunks are returned as a generator too).
ArticlesStatSales and AccountStatOperations use this new method to
avoid issuing #scale_chunks queries.
ArticleStat:
- fixed on Chrome
3f4a1adbb9