Add relative operations to gauges

The etsy statsd implementation includes support for relative operations on gauges (see https://github.com/etsy/statsd/blob/master/docs/metric_types.md#gauges). I've attempted to duplicate this functionality.

I followed the [statsd specs](https://github.com/b/statsd_spec#gauges) and retained the gauges as uint64 values.

This is my first attempt at coding in Go. Please be gentle!