[#24] unify logging to console

refacto every message to the console to use jirac_log for consistency
add a log-level option to set a level of details for JIRAC logs to the console : 0 = ERROR, 1 = INFO, 2 = DEBUG
option silent-mode is now implemented as setting the log level to -1 (ie. no log at all)
remove JIRAC header and victory ASCII arts because they are no logs and can not be controlled with log-level
refacto all program exits paths to use the same functions
move exit functions to core_functions.sh for consistency (ie. with the other log and exit related functions)

en bonus, j'ai fait une passe de correction d'indentation et d'usage des doubles bracket.
j'y ai pensé mais j'ai constaté qu'en pratique on n'avait jamais de warning, soit des logs INFO/DEBUG soit des logs ERROR après lesquels le programme exit. Donc je n'ai pas mis de niveau WARN.