[TIMOB-17489] Do not process data returned by callbacks

If a JavaScript callback - e.g. via Ti.*.addEventListener('event', fn); or Ti.Network.createHTTPClient({ onload: fn }); - returns a value, that value will be converted to a native object. If this is a large object this can crash the app, like demonstrated by the first button in this [example app](https://github.com/wappzapp/whatsup). I can't think of any use-case why the proxy needs the value the callback returns. So if there is a way to keep Kroll from converting it, this would help to prevent people from spending days - like we did - on nailing the crash. I'll publish a [post on tiDev.io](http://www.tidev.io/2014/07/08/dont-let-callbacks-cross-the-bridge/) at 5pm UTC explaining this issue.