fix deprecations of sizeWithFont for ios 7

[ioboi]

now the code doesn't have any deprecation warnings for ios 7.
i think it would be fine if you create a branch or tag for ios < 7.

cheers :)

[instantkaffee]

    CGSize size = [label sizeWithAttributes: @{NSFontAttributeName: self.labelFont}];

[sketchytech]

Firstly, excellent work. Thanks very much for contributing this code. In iOS 7 the sizeWithFont warning can be fixed with the following code:

sizeWithAttributes:[[self.labelFont fontDescriptor] fontAttributes]

Not suitable for earlier versions of iOS.