Clarify that EMPER is LGPL-3+ licensed and add license check

LGPL-3

LICENSE

Makefile

apps/Main.cpp

apps/WorkerSleepExample.cpp

emper/Actor.hpp

emper/BinaryPrivateSemaphore.cpp

emper/BinaryPrivateSemaphore.hpp

emper/Blockable.hpp

emper/Common.cpp

emper/Common.hpp

emper/Context.cpp

emper/Context.hpp

emper/ContextManager.cpp

emper/ContextManager.hpp

emper/CountingPrivateSemaphore.cpp

emper/CountingPrivateSemaphore.hpp

emper/Debug.cpp

emper/Debug.hpp

emper/Dispatcher.cpp