`selected-class` logic rewritten to reuse $addClass, $updateClass functions.

Small code cleanup PR. 
+ After we moved to ngModel `$scope` object is not used by `ngSelectController` anymore. 
+ `select-class` is an extended version of `ngClass` so I updated `select-class` implementation to match `ngClass` for more similar behavior. I hope one day we will be able to reuse some `ngClass` functions there. See https://github.com/angular/angular.js/blob/master/src/ng/directive/ngClass.js#L32-L59 for details.