Add support for iOS7.

NSString+KBAdditions.h

@interface NSString (KBAdditions)
- (CGFloat)fontSizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size minimumScaleFactor:(CGFloat)minimumScaleFactor;
@end

NSString+KBAdditions.m

#import "NSString+KBAdditions.h"

@implementation NSString (KBAdditions)

// Original code from http://stackoverflow.com/a/4383281/463892 & http://stackoverflow.com/a/18951386
- (CGFloat)fontSizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size minimumScaleFactor:(CGFloat)minimumScaleFactor {
    CGFloat minimumFontSize = [font pointSize] * minimumScaleFactor;
    CGFloat fontSize = [font pointSize];
    
    NSAttributedString *attributedText = [[NSAttributedString alloc] initWithString:self
                                                                         attributes:@{NSFontAttributeName: font}];
    CGFloat height = [attributedText boundingRectWithSize:(CGSize){size.width, CGFLOAT_MAX}
                                               options:NSStringDrawingUsesLineFragmentOrigin
                                               context:nil].size.height;

    UIFont *newFont = font;
    //Reduce font size while too large, break if no height (empty string)
    while (height > size.height && height != 0 && fontSize > minimumFontSize) {
        fontSize--;
        newFont = [UIFont fontWithName:font.fontName size:fontSize];
        
        attributedText = [[NSAttributedString alloc] initWithString:self
                                                         attributes:@{NSFontAttributeName: newFont}];
        height = [attributedText boundingRectWithSize:(CGSize){size.width, CGFLOAT_MAX}
                                                      options:NSStringDrawingUsesLineFragmentOrigin
                                                      context:nil].size.height;
    };
    
    // Loop through words in string and resize to fit
    for (NSString *word in [self componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]) {
        CGFloat width = [word sizeWithAttributes:@{NSFontAttributeName:newFont}].width;
        while (width > size.width && width != 0 && fontSize > minimumFontSize) {
            fontSize--;
            newFont = [UIFont fontWithName:font.fontName size:fontSize];
            width = [word sizeWithAttributes:@{NSFontAttributeName:newFont}].width;
        }
    }
    return fontSize;
}

@end

UILabel+KBAdditions.h

@interface UILabel (KBAdditions)
- (void)sizeToFitMultipleLines;
@end

UILabel+KBAdditions.m

#import "UILabel+KBAdditions.h"
#import "NSString+KBAdditions.h"

@implementation UILabel (KBAdditions)

- (void)sizeToFitMultipleLines {
    if (self.adjustsFontSizeToFitWidth) {
        CGFloat adjustedFontSize = [self.text fontSizeWithFont:self.font constrainedToSize:self.frame.size minimumScaleFactor:self.minimumScaleFactor];
        self.font = [self.font fontWithSize:adjustedFontSize];
    }
    [self sizeToFit];
}

@end