Verify membership when the dn is in the direct list of members for a group.

This saves us from sending a lot of requests for direct members of a group.
Merge pull request #22 from github/fix_is_direct_member

Verify membership when the dn is in the direct list of members for a group.
Merge branch 'master' into support_posix_groups

* master:
  Verify membership when the dn is in the direct list of members for a group.