Allow port to be string, prevent silent failures

In setup, if passed port is not an integer, attempt to parse it with
Integer/parseInt. Without this, if port was specified as a string,
the library will silently fail when instantiating DatagramPacket and
nothing gets sent to the statsd server.

Allows for more flexible configuration, ie
  (apply s/setup (clojure.string/split "localhost:5725" #":"))
Merge pull request #7 from loganlinn/port-parse-int

Allow port to be string, prevent silent failures