 - inspect the queue (some kind of "advanced peek")
 - use Boost C++ (or C++11) to improve the efficiency
 - change default behavior of `seize` and `release` (for version >=4.0.0, see issues #25, #27)
 - implement batch/separate activities
 - implement a parallel branch activity
 - del_generator, del_resource
 - add[del]_container (see simpy.resources.container)
 - add[del]_store (see simpy.resources.store)
 - monitoring: start/activity/end time per resource
