New options

I've add new option to rating_for_user, rating_for which is :readonly option. This option show rating in readonly mode. Also i have add :can_change option to letsrate_rateable method, which is can allow users to change rating after rate. In previous version if user try to change rating then "User has already rated." exception will be raised. Also i have change associations methods call, because in rails 3 it throw exception "invalid arguments"