additional gauge functionality -- deltas

Three enhancements:

1) Added support for gauges with scalar deltas as described here:

https://github.com/etsy/statsd/blob/master/docs/metric_types.md#gauges

foo:+50|g ==> "add 50 to current value of foo"
foo:-10|g ==> "subtract 10 from current value of foo"

2) Related to #1, fixed defect in existing code where recordGaugeValue("foo", -20) will be sent to statsd such that it will be interpreted as "subtract 20 from current value of foo", even though the the intent of the method is clearly "set the value of foo to -20"

3) Removed final modifier on classes to make it easier to extend functionality without modifying source