Polling and Last-Event-ID issues with IE

[izabellagal]

We have encountered two issues when using the plugin with IE 9 and IE 10.

1. We have observed that when an Ajax call times out because there are no new events, the source in not queried again.
   We have also observed multiple parallel requests because the timer set to poll the source again after processing the events from the current request is not fired once, but multiple times.

   We have refactored the code to poll again into a separate method and we have added an error handler for the Ajax call, handler that calls this refactored method.

2. When the fall-back implementation is used, the Last-Event-ID request header is not set. Due to this, our server application sends all the events, both new and old, on each request.

   To resolve this problem we have made a change to get the lastEventId value from the parsed data and we have added the Last-Event-ID header to the Ajax call.