Cancellable changes, es6 set support, diff changes

• Add cancel feature (onDerives can throw a cancel to have a change cancelled) #50
• Add diff #51
• Add support for es6 Sets
• Let branchAll() and renderAll() work on objects, es6 maps and es6 sets
• Add .key
• Add details to change and derive callbacks
• Fix #45
• Chained change callbacks now all occur after derive

No breaking changes.