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!
You can simplify this to (note the `'` rather than `"` to indicate a rune, not a string):

```go
if val[0] == '+' || val[0] == '-' {
```

But I think in this case a `switch` statement is actually preferable, it would read:

```go
switch val[0] {
case '+', '-':
    // ...
default:
    // ...
}
```

Cool?