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!
... the rest of the code that follows will be easier to read and won't continue to type assert (I'm not sure how the compiler optimizes this, honestly).