jsonp-polling does not work in socketio extension

The current `JSONPPollingTransport` implementation

1. does not escape the JavaScript string before inserting it into the JSONP response
2. inherits `handle()` from `XHRTransport`, which only checks the request body (for jsonp-polling, the data is sent in a parameter `d` ([source](http://showmetheco.de/articles/2011/8/socket-io-for-backend-developers.html)))