Ephemeral udp with port reuse AND with exiplicit close

This adds to pull request #8 a .close() function that lets you explicitly shut down bound ports.  Using this you can either use the ephemeral bound feature, or pass in a port yourself and let node-statsd manage it, to reclaim when .close() is called.

This lets servers shut down quicker and cleaner upon non-fatal signal or whatever.  You can merge this and close both issue #6 and issue #8 if so inclined.