Rewrite mergeKeySets to be O(n) instead of O(n^3)
Fix #874: Edit thinking-in-react

Accidentally pulled #874 into gh-pages.
Fix #845, Trivial year change

This was accidentally pulled into gh-pages.
Always call callback in component context

This brings these other call sites in line with line 67 of ReactUpdates.js:

callbacks[j].call(component);
Merge pull request #876 from spicyj/cb-ctx

Always call callback in component context
Merge pull request #864 from spicyj/transition-key-set-faster

Rewrite mergeKeySets to be O(n) instead of O(n^3)
Use fallback data for composition events if necessary
Improve "Can only mount when umounted" message
Merge pull request #878 from spicyj/mount-unmounted

Improve "Can only mount when umounted" message