More modularity

-Function “showMessage” is now prepared to accept NULL valued pointers,
which brings us to…
-Removed from functions like “process” the need to print NULL if
there’re no more messages
-Swapped operation -= 1 with usual decrementing
-Small visual edits
More modularity 2

TESTED: with examples, public tests and private tests
-Added function “deleteLastMessage”.
-Implemented function mentioned above in “process” and in “kill”
-Updated kill: now only needs one argument
-More code organization