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.
this assignment is really tough to read, and I think is better written with a ternary
```js
        var eventHandlerName = listener.__reactBoundContext ?
          listener.__reactBoundContext.constructor.displayName :
          '<<anonymous>>';
```