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.
Thanks for the tip.
However your (or Jasper's) original code is using the "if x is 'blah'" statements all over the place. Shouldn't you change that as well then?