Add suPHP_UserGroup directive to directory context

suPHP_UserGroup is only supported in per-direcctory context. We add this
directive analogous to the per-vhost suphp_ directives, by making sure
that suphp_engine is actually enabled.

  suphp => { 'user' => 'myappuser', 'group' => 'myappgroup' }

With documentation and tests in place, this should fix #451