Relative 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!
It looks like the code already did this, but none of these `ERROR` logs need to `string(val)` either - passing a `[]byte` to a `printf` format `%s` is perfectly OK.