declineOffer() should be added to Python-C++ bindings and used in test_framework.py

Came across this while trying to call driver.declineOffer()

Traceback (most recent call last):
File "./nmpiexec.py", line 76, in resourceOffers
driver.declineOffer(offer.id)
AttributeError: '_mesos.MesosSchedulerDriverImpl' object has no attribute 'declineOffer'

It should involve a quick fix in src/python/native/mesos_scheduler_diver_imp.cpp

declineOffer() should also be used in src/examples/python/test_framework.py, which currently holds offers that it should decline if tasksLaunched >= TOTAL_TASKS