Advanced styling support with text attributes for kerning, shadow, etc.

Hey,

I've added support for advanced styling. HMSegmentedControl can now be adjusted for any custom design I can think of. In my case I needed kerning but stuff like shadows and other text attributes are now customisable.

I've dropped support for iOS 6 and 7 though for the sake of simplicity: I would introduce a significant flaw by leaving the code for compatibility with the macros anyway — one would get get the old behaviour on iOS 7+ if they supported iOS 6 and below.