Added support for `su` option introduced in logrotate 3.8.0

As requested by issue #14, added `su` support introduced in logrotate 3.8.0.  This introduces three new `logrotate::rule` parameters, `$su`, `$su_owner`, `$su_group` and are modeled after the existing `$create*` options for consistency.  Also added full testing and passed `script/cibuild`.

See: http://svn.fedorahosted.org/svn/logrotate/tags/r3-8-0/CHANGES