Only stop if the transition on this element ended

This way it doesn't end when some random element elsewhere on the page has an ending transition.

It must be noted that this is not a complete fix. If you have multiple transitions on the element, it still ends when the first ends.

I ran into this not working (the transition ends almost immediately, because some unrelated transition ends). This fixes it for me, but I was wondering whether this cornercase was considered, and whether the other cornercase (multiple transitions) was considered.

I think it's very hard to get this correct, and think it might be better to make the timeout explicit, or at least allow to set it explicit, and fall back to this behaviour. But I'm really interested in feedback on this one.
Guard against contentEditable with children props
Added event parameter
Make sure it works when we have no css transitions
add picture element and related attributes
add picture to xjs.knownTags
Add support for <dialog> tag.

fixes #2017
Throw an error when functions on `statics` clash due to duplicate keys
More informative error message for mergeObjectsWithNoDuplicateKeys
Abbreviated error message
Throw if a reserved property is defined in 'statics'
Deprecate 'return false' in event handlers
Re-add  with warning and monitorCodeUse
Fixed style and comment nits
Test for both true and false
More info in monitorCodeUse call
Look on __reactBoundContext for displayName
Add 2048-react to open-source demos

Nothing too complicated, but I think it could be a fun beginner demo.
Update Object.assign polyfill

The spec changed to allow null & undefined sources without throwing.
Now our code depends on this new behavior. The only browser that
implements Object.assign has also updated its native implementation to
this behavior (Firefox Nightly).
Make `npm run-script jest` available

This is slightly shorter than typing `./node_modules/.bin/jest`.