[#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)
inline conditions, fix indent in few places, remove useless [[]]
[[]] actually required for == operator with pattern matching
[#5] Display warning if no scm.url

Previously we generated comments'links based on
scm.url OR scm.connection. Now we only rely on
scm.url for it represents the publicly browsable
url of the repository.
If absent we log it as a WARNING but we still
generate a comment (without links).