Make member_filter method take a Net::LDAP::Entry

This is in order to update it to filter by DN and CN.
Include memberUid=CN in member_filter
Add test for posixGroup handling for Domain#is_member?
Handle Net::LDAP::Entry[] returning an Array, filter nesting

Since moving away from "entry.dn" to "entry['memberUid']", the filter
value was changing and breaking the filter generated.

Also, Net::LDAP::Filter can't handle generating a filter with more than
two filter conditions (aka "(|(one)(two)(three))"), so account for that.
Exclude memberUid members from Group#members

This explicitness prevents breaking behavior when calling super
for PosixGroup#members.

Not all membership names are created equal, it seems.
Test posixGroup with/out recursive fallback

Since this exercises two different code paths and needs
full test coverage.
Define posixGroup fixture data, schema
Use custom schema for posixGroup tests
Merge branch 'master' into posixGroup-membership
:fire: a bit more
Match memberUid to configured uid field

Reverts changes to memebership names, favoring explicit filter conditions.
Use reduce(:|)
Accept memberUid uid attribute as parameter, better doco
Merge pull request #33 from github/posixGroup-membership

posixGroup membership queries from Domain