This commit consists of the implementations of BroadcastSwitch::Handle(Broadcast*) and the forwarding functionality in Scheduler, in addition to some minor
modifications to accomodate these changes.  I further changed the private members of classes in the entities hierarchy to protected and added a virtual method to
Entity to handle Broadcasts.  Lastly, this commit contains a very small but real simulation: simulation2.cc.

At this point, simulation of broadcasts is complete except:

1) Input (ex. network topology) is inlined in the code rather than fed into the program via a file.

2) A switch is currently not inferring anything about the network from the broadcasts it receives.

3) Timers are not implemented.