Add Message constructor. Add message specific sem in PCB. Add various
implementation of Process class methods

PCB.message_semaphore_specific is for processes, which wants to read
message from specific sender. If specific sender isn't found, it call
wait_p on this semaphore to block process. Only first new message call
signal_p, because that semaphore is used as boolean with blocking
function..
The following methods are implemented:
- run()
- stop()
- create_process()
- process_success()
- process_error()
- read_message() -- both partly