在使用AutoLayout后，想要使用代码控制ScrollView的contentSize，应该在[super viewDidAppear:animated]之后进行。
http://willsbor-blog.logdown.com/posts/179101-ios-autolayout-at-scrollview
http://www.2cto.com/kf/201312/267804.html
http://stackoverflow.com/questions/19212877/scrollview-not-working-in-ios7-with-autolayout