feat($ionicScrollDelegate): rememberScrollPosition, scrollToRememberedPosition

/**
 * @ngdoc method
 * @name $ionicScrollDelegate#rememberScrollPosition
 * @description
 *
 * When this scroll area is destroyed, its last scroll position will be
 * saved using the given id.
 *
 * @param {string} id The identifier for this saved scroll position.
 */

/**
 * @ngdoc method
 * @name $ionicScrollDelegate#scrollToRememberedPosition
 * @description
 *
 * If a scroll position was remembered using the given id, loads the
 * remembered scroll position and scrolls there.
 *
 * @param {string} id The identifier for this saved scroll position.
 * @param {boolean=} shouldAnimate Whether to animate the scroll.
 */