Workers should cleanly shutdown quickly

The worker handles SIGHUP and SIGTERM, but it only affects the
job polling loop and not the processor. This patch passes the
stop notification to the processor so it can set the job to
TIMED_OUT and quickly and cleanly shut down rather than waiting
for the current job to finish processing.
Merge pull request #303 from rackerlabs/rm5870-graceful-shutdown-rebase

Workers should cleanly shutdown quickly