[#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.
[#1] display selected commit hash and subject in INFO in print-mode 1