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.
please use comparison statement here
http://stackoverflow.com/questions/26595/is-there-any-difference-between-foo-is-none-and-foo-none