report_down when stopping ZK reporter.

Running nerve within a docker image using ZK as a reporter currently
does not allow one to automatically deregister on shutdown.

However, the etcd reporter does have this behavior so we should be
consistent.

Add a test for the expected lifecycle of a ZK reporter.
Merge pull request #64 from narbsy/deregister-from-zk-on-exit

report_down when stopping ZK reporter.