*_floor functions do not perform daylight savings time adjustments

[lorrin]

In the following example, year_floor returns a January 1st datetime that still has daylight savings time enabled.

>>> from datequant import year_floor
>>> from pytz import timezone
>>> from datetime import datetime
>>> pst8pdt = timezone('PST8PDT')
>>> pst8pdt.localize(datetime(2013,1,1))
datetime.datetime(2013, 1, 1, 0, 0, tzinfo=<DstTzInfo 'PST8PDT' PST-1 day, 16:00:00 STD>)
>>> year_floor(pst8pdt.localize(datetime(2013,7,1)))
datetime.datetime(2013, 1, 1, 0, 0, tzinfo=<DstTzInfo 'PST8PDT' PDT-1 day, 17:00:00 DST>)