game of life: create new branch graphics

Starting point to develop a graphical interface.
game of life: make things ready for SDL use
game of life: add graph.c/.h to Makefile
game of life: add license to graph.c/.h
game of life : add SDL functions

Signed-off-by: Romain Gauthier <rj.gauthier@laposte.net>
game of life: update main to enable graphics capabilities
game of life: add event listener

Adding the event listener enables the user to close the window directly
through the close button. It also keeps the program from not responding
(which can be annoying, depending on the graphical interface).
game of life: add graphmode option

The --graphmode option enables the user to choose between SDL graphical
interface and the terminal to display the patterns.
Merge branch 'graphics'