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.
Instead of removing this completely now, we should make a release where this still works but it gives a warning (in `__DEV__`) using console.warn – if you search around you should see other examples of this. It's also good to add a monitorCodeUse call which does nothing in the open-source build by default but makes a log internally at Facebook (and theoretically outside as well for sophisticated clients – I've been meaning to set it up at KA but haven't had a chance yet).

After people have had a chance to upgrade, we can drop it completely.