adding gauge support

* based on https://github.com/lookout/statsd
  initial implementation
* prefix set to `stats.gauges` as in the "canonical" (etsy)
  implementation.
  see https://github.com/etsy/statsd/blob/master/backends/graphite.js#L196
updated README with gauges section
the etsy implementation does not clear gauges

Once a value is reported as a gauge, it keeps getting
reported until another value overwrites it
Merge pull request #6 from gingerlime/master

adding gauge support