Workaround for font name handling change in iOS 7.

Font name handling changed in iOS 7. The assertion below will pass in pre-iOS7 installs but fail in iOS 7. This pull request is a workaround for the new behavior.

```
UIFont *uifont = [UIFont boldSystemFontOfSize: 14.0f];
CGFontRef cgfont = CGFontCreateWithFontName((__bridge CFStringRef)[uifont fontName]);
NSAssert(cgfont, @"could not create cgfont from bold system font name");
```

The behavior of both the old and the new approach should be the same, but the new approach is only used in iOS 7 just to be on the safe side.