Update to MIT license
try to cut down on ping port traffic with health check interval
sweep every 5 instead of 1 and check backend before frontend and local_queue in between
shift off of local queue for ordered processing and concat instead of + for less copy
constantize the empty string
add empty to broker
let server shutdown the context when using a shared context via inproc
Merge pull request #217 from localshred/abrandoned/ping_port_health_check

Abrandoned/ping port health check
bump to 3.3.3
Use a job_queue to pass jobs to workers

This streamlines the flow of work from the broker to the worker. Rather
than having to balance a local queue with work coming in currently, all
work now goes directly to the queue. The workers pull messages off the
queue in the order they were received.
Fix transmission and shutdown

Switched to a dealer socket in the workers so they can send multiple
replies without waiting for a response. Because new work is coming in
through the queue, they no longer need to receive data from the broker
on the zmq socket.

The server was hanging on shutdown. I'm not sure why, but I suspect
reading from the queue was blocking it. I updated it to use the queue
itself to signal shutdown. The worker will also propogate the shutdown
signal when it receives it, so the signal will continue to propogate
until all workers shutdown. That's the theory at least.
Be more pedantic about shutdown coordination
Let the server handle inproc context
Use constance for an empty string
Prevent stalling on frontend
Allow workers to respond directly through a mutex
Integrate nice ping port health check
Pass the broker rather than it's thread to workers
Fix runtime error
Wrap frontend reads in a mutex