functionality to filter and exclude metrics

The statsd instance I am using is handling more stats than needed to be sent to librato because they were being monitored through graphite.  In order to only send the stats I want in Librato I added functionality which allows for a combination of metrics to be filtered out and/or to be specifically excluded from being sent.  This helps give greater control over of what is sent so charges aren't incurred for metrics that aren't being used through the Librato interface.