Vertical text cropping

If the label bounds are not high enough, `CTFramesetterSuggestFrameSizeWithConstraints` returns `CGSizeZero` and no text is rendered at all. By setting the text rect height to be twice the font point size, `CTFramesetterSuggestFrameSizeWithConstraints` doesn't return `CGSizeZero` thus the text is rendered cropped.

Legend:
> Black text = `UILabel`
> White text = `TTTAttributedLabel`
> Static = text set in the nib
> Dynamic = text set programmatically

Rendering before the vertical cropping patch:
![TTTAttributedLabel-original](https://f.cloud.github.com/assets/51363/50224/fd4a9b7c-5999-11e2-856c-2cfc9f8f142f.png)

Rendering after the vertical cropping patch:
![TTTAttributedLabel vertical-cropping](https://f.cloud.github.com/assets/51363/50225/fd4de980-5999-11e2-9361-f1023334cfa4.png)