Multiple improvements

[elboletaire]

I open the PR, but will let it open for some time, until we're sure everything works as expected.

Changes with this PR:

• Now there's no need to unset CI_REF_NAME when publishing, just set your mode to publish #17
• Set failure_notification_mode to exit-code by default #13
• Automatically define sonar variables based on gitlab ci ones (if they are not defined) #10
• Add documentation about sonar-project.properties file #14
• Move all the -Dsonar.gitlab. params to the conditional block for preview mode, as it's the only place where gitlab params are needed.
• Created an alias of the bash file, so you can now use gitlab-sonar-scanner as the executable a part from the old way, sonar-scanner-run.sh.
• You can now pass additional params to the executable, like so:

script:
- gitlab-sonar-scanner -Dsonar.custom.param=Whatever