Add InverseOffer to C++ Scheduler API.

The initial use case for InverseOffer in the framework API will be the maintenance primitives in mesos: MESOS-1474.

One way to add these to the C++ Scheduler API is to add a new callback:

  virtual void inverseResourceOffers(
      SchedulerDriver* driver,
      const std::vector<InverseOffer>& inverseOffers) = 0;

libmesos compatibility will need to be figured out here.

We may want to leave the C++ binding untouched in favor of Event/Call, in order to not break API compatibility for schedulers.