Fixed UILabel and NSString size calculations for >iOS7

All UILabel and NSString category methods now return a CGFloat instead
of a float

- heightForWidth, - widthForHeight and similar methods now use -
boundingRectWithSize:options:context: to calculate sizes

Removed (deprecated) - setMinimumScaleFactorIfAvailable: and -
setAdjustsLetterSpacingToFitWidthIfAvailable: