fix rpc server._handler = None error by using Conditional variable
Merge pull request #25 from fegin/master

Add write expression.
ignore the messages until the socket.start_listening() been called
changed listen variable/method name.
Merge pull request #24 from EasonLiao/master

fix rpc server._handler = None error by using Conditional variable
worker exits after heartbeat message timeout.
Merge pull request #28 from EasonLiao/master

worker exits after heartbeat message timeout.
Have watchdog listen on stdin for messages.

Stdout appears to always work.
Merge pull request #29 from spartan-array/fix-watchdog

Have watchdog listen on stdin for messages.
Make sure to shutdown cluster when controller goes away.
Merge pull request #30 from EasonLiao/shutdown-on-master-del

Make sure to shutdown cluster when controller goes away.