Add graphite sink for metrics

This adds a metrics sink for graphite.  The sink must
be configured with the host and port of a graphite node
and optionally may be configured with a prefix that will
be prepended to all metrics that are sent to graphite.
Cleanup GraphiteSink.scala based on feedback

* Reorder imports according to the style guide
* Consistently use propertyToOption in all places
Merge pull request #151 from russellcardullo/add-graphite-sink

Add graphite sink for metrics

This adds a metrics sink for graphite.  The sink must
be configured with the host and port of a graphite node
and optionally may be configured with a prefix that will
be prepended to all metrics that are sent to graphite.