fix: weak delegates to prevent leaks

[gbuela]

Strong delegate references are creating reference cycles. This isn't noticeable with the Example app because there's a single view controller. But if you'd use Pikko in a VC that was presented as modal or pushed into a navigation stack, the memory debugger would show that instances of the VC, Pikko, HueView and BrightnessSaturationView never go away due to the cycle created by the delegates.

[Alipacman]

I also noticed this, thanks!