iOS7でNSString#sizeWithFontがDeprecated

代わりにsizeWithAttributesが使えるようだ。

CGSize size = [s sizeWithAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:20.0f]}];

【参考】
http://stackoverflow.com/questions/18897896/replacement-for-deprecated-sizewithfont-in-ios-7