Generate JSON string using NSJSONSerialization

I'm using this plugin to implement push notification things for my phonegap app, and found this does wrong behavior.
Original code is erroneous for these two reasons.

1. Flatten all nested objects. It should keep the original JSON structure from APN.
2. Escaping code using `stringByReplacingOccurrencesOfString: withString:` causes crash when `thisObject` isn't type of `NSString`.

I think using iOS's native JSON library is good for simple and reliable code, so I commit this hunk.

Take a look at this, and consider merge this one. But this hunk doesn't flatten the nested dictionaries, it can cause compatibility problems for other guys.

Thanks.