Deprecate 'return false' in event handlers

Fixes #2015. Wasn't sure how to handle `react-version` in _config.yml, given that this is a v0.12 milestone.
Sorry, missed this: internally, this is a different function that logs the message. You'd pass a second parameter, which is an object that contains all you need to know to trace back to this function so that people can act upon it:

```
monitorCodeUse('react_event_return_false', {eventHandlerName: listener.__reactBoundMethod.displayName});
```

And maybe some other info, dunno. See #2052