[WIP] feature/animation

[seanclen]

Add custom animation support. From #1

Initial thoughts:

• Separate drawing into separate subclass of CALayer
• Properties of CALayer subclass to animate are properties in parent UIView class
• Easy way out: use a mask.
• Future-proof way: animate by stepSize. (opportunities to play with width, color, etc.)

[philipengberg]

Very exciting 😃 When I get time later this week, I can share my current implementation with you here. Would love to share some feedback 👌