build: add a simple Makefile.

Adds a simple build process based on a Makefile with output in a separated
directory.

make all: builds all classes
make clean: clean build products
make serveur: builds and runs the server part (in background)
make client: builds and runs the client part (blocking process)
make run: starts the serveur in background, then the client
Merge pull request #1 from djod4556/master

build: add a simple Makefile.