Added ability to pass a callback in to the Socket.close method.

This callback is passed in to the server.close method to be called when
the server closes down successfully. Mimics the functionality in
Socket.listen -> server.listen.
Merge pull request #78 from james-huston/master

Added callback to Socket.close to mimic Socket.listen.
refactor previous commit
Merge pull request #80 from gjohnson/master

Clean up message arguments
refactor pubsub.subscriptions test to match string version
update benchmarks. Closes #72
remove batching

increases throughput and decrease latency for realistic msgs/tick,
we can revisit this in the future if/when it is a real problem
add HWM support. Closes #19
Release 0.5.0
add better HWM example
add exit() to HWM example