Added support for multiple objectClasses to be set on the EntityMap.

Currently, a user extends EntityMap and sets, amongst other things, $this->ldap_object_class which expects a string. I am not sure how common it is but in our active directory we have multiple objectClasses and when it came to adding a new user, I needed to set a number of objectClasses for the operation to complete successfully.

This pull request preserves existing functionality in that a user can still set $this->ldap_object_class with a string value but also allows a user to set an array of objectClasses if they so wish.