Improved the way we display the executed isntaller command

This PR tries to solve #80 for the most used case.  Around 80% of users use Linux or Mac and most of them will install the installer in `/usr/local/bin/`, so we can easily improve commands' output.
We could be even smarter and check if `dirname()` of `$_SERVER['PHP_SELF']` is one of the paths in `$_SERVER['PATH']`. What do you think?