Support static nested groups in LDAP

This refers to groups that have member: <another group DN> as an attribute
- Add in a utility method in the SpringSecurityLdapTemplate to retrieve multiple attributes and their values from an LDAP record
- Make the DefaultLdapAuthoritiesPopulator more extensible
- Add an LdapAuthority object that holds the DN in addition to other group attributes
- Add a NestedLdapAuthoritiesPopulator to search statically nested groups
Possibly make protected getters final