Modify `scrollbar` directive

* I modified `scrollbar` directive.

    * This directive now applies perfect-scrollbar plugin in an
      `$apply` block.
    * This directive now updates the scrollbar control when an
      AngularJS event named `update` was emitted in its scope.

* I removed `repeatAndUpdate` directive because this directive is not
  needed anymore due to the change in `scrollbar` directive.