0.9.x dst fixes

Fix for  DST issues in 0.9.x (issue #810)
Backport of 
https://github.com/brutasse/graphite-api/commit/c8854f584b4ba216f9706fa5bf42597a4f9c969c
https://github.com/brutasse/graphite-api/commit/a0ae47023cce8fe7dc2815c8ed46ffc16f3bdefe
to 0.9.x.
It works for me, I switched DST back and forth, but not sure if it fix initial issues. :)
Same, with `django.utils.timezone` you can do `timezone.now().replace(hour=hour,minute=min,second=0)`.