NSMutableAttributedString setFont functions crash with iOS 6 drawing functions

Just a heads-up.

NSAttributedStrings with the font attribute set using the old Core Text functions (which the methods in the NSAttributedString attributes category included with this project use), when drawn using the new iOS 6 NSAttributedString drawing functions, will crash.

```
NSMutableAttributedString* foo = [NSMutableAttributedString attributedStringWithString:@"foo"];
[foo setFontName:@"Helvetica" size:13];
[foo boundingRectWithSize:CGSizeMake(320, 50) options:0 context:nil]; // CRASH
```

It appears that Apple assumed that the attribute value would be a UIFont object, when the documentation for the Core Text font attribute (still) says that it needs to be a CTFont object.

Although this is an iOS bug and not you guys' fault, you should probably still keep this in mind.