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!
actually, now that I'm following what's going on here... I think we can change all of this code at the top of this section doing all this initialization to a single line:

```go
gaugeValue, _ := gauges[s.Bucket]
```

That means the separate declaration and this `if/else` block would go away.