Revisit root path to better handle chrooted installs

Also, allow for the administrator to manually define the ROOT_PATH in the
config file (the very last mile).
Document osTicket::get_root_path()
Merge pull request #716 from protich/feature/root_path_revisited

Feature/root path revisited -- hopefully this issue will die here

Reviewed-By: Jared Hancock <jared@osticket.com>