Animate the picker when showing/hiding
Change each slider’s background when the current color changes
Add a UISlider subclass that takes care of the gradient backround
Refactor the picker using the new UISlider subclass
Add a method to the UISlider subclass for returning human readable text for each color component

In the future this will be useful when there is a need to use different ranges (like for hue or alpha).
Merge branch 'feature/slider_background' into develop