Add implementation of `diffPast` operator.
Add implementation of `diffPast` operator.

reactors-io/reactors,reactors-io/reactors,storm-enroute/reactive-collections,storm-enroute/reactors,storm-enroute/reactors,storm-enroute/reactors,reactors-io/reactors,reactors-io/reactors,storm-enroute/reactors