实例化alertcontroller之后，可以遍历view的子视图，子视图title和message嵌套了多层父视图，（需要遍历6层subViews），
下面是取title和message的父视图的代码：
UIView *subView1 = alert.view.subviews[0];
UIView *subView2 = subView1.subviews[0];
UIView *subView3 = subView2.subviews[0];
UIView *subView4 = subView3.subviews[0];
UIView *subView5 = subView4.subviews[0];
取title和message：
UILabel *title = subView5.subviews[0];
UILabel *message = subView5.subviews[1];
然后设置message内容居左：
message.textAlignment = NSTextAlignmentLeft;
