PearlOverlay cancellation & a bunch of helpers.

[ADDED]     Support for handling cancellation of the PearlOverlay with a block.
[FIXED]     Missing New and Old keys in notification for -observeKeyPath.
[ADDED]     scheduled equivalent to block-based timer creation + get rid of block holder and association.
[ADDED]     NilToNSNulls, NSNullToNils: plural forms of their existing singulars.
[ADDED]     PearlHashCode, PearlHashFloat(s): Tools for easy -hash implementation.
[ADDED]     PearlTween: Made PearlCCAutoTween generic: tween any property!