add handleCallback to M.Object

[FrankStierle]

Please add this often used method right after bindToCaller

/**
 * Calls a method defined by a handler
 *
 * @param {Object} handler A function, or an object including target and action to use with bindToCaller.
 * @param {Object} arg One or more arguments. If more, then apply is used instead of call.
 */
handleCallback: function(handler, arg) {
    if (typeof(handler) === 'function') {
        handler(arg);
    } else if (handler && handler.target && handler.action) {
        var action = typeof(handler.action) === 'function' ? handler.action : handler.target[handler.action];
        var call   = this.bindToCaller(handler.target, action, arg);
        call();
    }
},