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.
Updating this PR would be fine, just add a new commit with the changes?