Adding :class option to load_resource so one can customize which class to use for the model - closes #17

Don't fetch parent of nested resource if *_id parameter is missing so it works with shallow nested routes - closes #14

Adding :collection and :new options to load_resource method so we can specify behavior of additional actions if needed.

turning load and authorize resource methods into class methods which set up the before filter so they can accept additional arguments

calling controller's load_resource and authorize_resource from load_and_authorize_resource to maintain backwards compatability, even though it's not the most efficient way

moving can definition into ability instance instead of class, this removes ugly instance_exec command