Added link up and link down events with the corresponding Handler methods
in Entity and Switch.  The parser has been appropriately updated.
Implemented Controllers and link status messages.  Moved the functionality
to track hearbeats into a separate class.  Added an analogous class for
link status messages.  Made FindInPort non-generic as all entities have ports.
Merge branch 'master' of https://github.com/NetSys/ddcsim

Conflicts:
	src/entities.cc
	src/entities.h
	src/events.cc
	src/events.h