Access all objects for a specific user

This commit is the first half of the issue discussed [here](https://github.com/1up-lab/OneupAclBundle/issues/11).
It adds the ```findObjectIdentitiesForUser``` method.

As requested I added some test, and added the ```TokenInterface``` support, but did not add the ```RoleInterface``` support because I don't think a Role can be have ACL on objects, but correct me if I am wrong. If so, the RoleInterface does not provides a ```getClass()``` nor a ```getUsername()``` method.