Bring perSecond from master -> 0.9.12

The feature at:

   https://github.com/graphite-project/graphite-web/commit/3877366221ee93bfccabce736d69a4b11da25ea5

is too useful to leave in master, since 0.9.x is currently still in use.
Merge pull request #548 from graphite-project/bring-persecond-to-0.9.x

Bring perSecond from master -> 0.9.12