Add support for organization units in LDAP group mapping

When doing a mapping on a ldap system, only mapping between xwiki groups and ldap groups works. If the client uses OU (organizational unit), the mappings won't work. I guess ldap search expect final objects (cn) and not containers.

It would be nice that xwiki ldap plugin accept OU and not only groups, because some client don't want to change their infrastructure.

Here some examples of ldap objects.

group:

dn: CN=groupeUK,OU=UK,DC=xwiki,DC=local
objectClass: top
objectClass: group
cn: groupeUK
member: CN=xwikierUK,OU=UK,DC=xwiki,DC=local
distinguishedName: CN=groupeUK,OU=UK,DC=xwiki,DC=local
instanceType: 4
whenCreated: 20110831163038.0Z
whenChanged: 20110831163111.0Z
uSNCreated: 49275
uSNChanged: 49279
name: groupeUK
objectGUID:: DaXGG64bNEmtRwAiIwNVgw==
objectSid:: AQUAAAAAAAUVAAAAtiexj46ADRfZte1gVwQAAA==
sAMAccountName: groupeUK
sAMAccountType: 268435456
groupType: -2147483646
objectCategory: CN=Group,CN=Schema,CN=Configuration,DC=xwiki,DC=local
dSCorePropagationData: 16010101000000.0Z

OU:

dn: OU=Users,OU=FR,DC=xwiki,DC=local
objectClass: top
objectClass: organizationalUnit
ou: Users
distinguishedName: OU=Users,OU=FR,DC=xwiki,DC=local
instanceType: 4
whenCreated: 20110831135324.0Z
whenChanged: 20110831135324.0Z
uSNCreated: 49202
uSNChanged: 49203
name: Users
objectGUID:: C/xNtm+5lkCj61qLZgqAsg==
objectCategory: CN=Organizational-Unit,CN=Schema,CN=Configuration,DC=xwiki,DC=local
dSCorePropagationData: 20110831135324.0Z
dSCorePropagationData: 20110831135324.0Z
dSCorePropagationData: 16010101000000.0Z