posixGroup membership queries from Domain

`GitHub::Ldap::Domain#is_member?` doesn't correctly validate group membership for posixGroups due to some assumptions by the `Filter#member_filter` about the DN.

This changes the `Filter#member_filter` method to accept a `Net::LDAP::Entry` object and maps the correct group type with the correlated member attr (which, for posixGroup, is the member entry's CN).

The [RFC](http://tools.ietf.org/html/rfc2307) doesn't say specifically that the `memberUid` needs to be the member entry's `uid` field but it certainly implies it. In practice, I've never seen it be anything except the `uid`, but the configured user id field is probably safest.