Add group domain class.

Search recursively for members and groups. Unoptimized version for LDAP servers that don't support virtual attributes.
Remove escaped_domains reference.
Get all groups under a tree.

This is used when we just want a plain list of groups. For example, for listing groups under the configured domain bases.
Initialize a group with its entry from the server.

Reduce the number of arrays created and memoize the member entries.
Allow to filter groups under a domain tree by CN.
Merge pull request #14 from github/recursive_ldap_members

Add group domain class.