Log request counts for Librato to pick up

We're looking to do some app health monitoring, and Librato seems like a
much easier integration than New Relic. Librato consumes specially-
formed log lines, whereas New Relic seems to require a complex Python-level
integration.
Log request duration in addition to count
Rename request_duration to response_time
Merge pull request #1570 from gittip/librato

Emit log messages for Librato to pick up