Added support for the new stats type "delta"

The counter (as increment) and raw stats type (as the counter) were implemented wrongly and when a gauge is sent as a raw stats type this will go wrong. Also the gauge will not allow us to calculate this back to rate/second easily, so therefore it is easier to solve this by adding the delta stats type.
You're right, good one! if you see the chance to improve this, let's do so at least.. :)