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. :)
Instead of `datetime.utcnow().replace(tzinfo=pytz.utc)` you can do

```python
from django.utils import timezone
timezone.now()  # returns the tz-aware current datetime
```

You should also add `USE_TZ = True` to `webapp/graphite/settings.py`