[#30] Use gtac instead of tac on osx

tac is not available out of the box so mac users must install
coreutils to get it.
Check that tac|gtac is available
[#2] implement option --standard-output

--standard-output option enables silent mode automatically, requires either option --number or option --grep and print generated comment to the standard output
also:
 - minor fix in help of --number option
 - add default values for all variables initialized from command arguments
 - minor indent fixes in ui_functions.sh
[#2] remove description override in print-mode 0

this functionnality makes print-mode 0 non pipable, it was not used, so it was decided to remove it
Add link to Gitter

Add a link to the gitter chat room to provide a better support
[#2] fix options failing if after --standard-output option

--standard-output had a shift command causing next commande line option to be swallowed
 ex.: commande 'jirac --standard-output -n 1' failed with "missing option --number" message
[#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
[#33] --standard-output now effectively change log level

--standard-output used to implictly enable silent-mode (or actually fail at doing it) but since errors are printed to stderr, there is no need to silence them, changing log level to display only ERROR is enough
[#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 DEBUG but we still
generate a comment (without links).
[#27] remove --print-mode option and keep only print-mode 1
[#36] remove linebreak before commit body in print-mode 1

Conflicts:
	jirac
[#39] unkown options of one char must display "unreconized option" msg
[#39] fix *_and_exit methods displaying log even with no argument

simplified code, remove support for multiple parameters in usage_and_exit and message_and_exit, jirac_log does not support it anyway
Revert "Add link to Gitter"

Following conversation on #32, we decided to remove the gitter's link.
This reverts commit de611684bd18b2e087d2db120ffefc38772a0b6f.
[Enhancement] Use subshell

Get rid off the bad patter cd somewhere; cd -
Instead use the subshell which runs into another
process, and thus affecting only its state and
not the calling script. Also remove the global
variable branch. Use echo instead.