Added support for `su` option introduced in logrotate 3.8.0

See: http://svn.fedorahosted.org/svn/logrotate/tags/r3-8-0/CHANGES
Added missing su option documentation to `README.md`.
Merge branch 'su_option_support' of https://github.com/dbeckham/puppet-logrotate into dbeckham-su_option_support