Finally able to solve? this: LiulietLee/Pick-Color#2
MKLayer’s superLayer observers
MKRefreshControl’s parentScrollView observers
MKSwitch’s parent MKSwitch

It’s sad that when containing MKLayer and MKRefreshControl, current
implementation requires calling `recycle` in `deinit`…

- Remove self, type delcaration, use lazy init, parameter same as
default
- Merge if-let, use optional changing for short ones, use implicit
unwrapped for guaranteed,
- Replace file private with private, provide public init for an open
class
- Update README