[#12]remove extra line

Remove return at beginning of generated comment
[#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
[#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).
[#5] Replace WARNING log level

Following arguementation on issue#5 we decide to pass the
level of log to DEBUG when there is no scm.url in the pom