Allow for POST requests, verbose=1, and the text of the IOErrorEvent to be sent

I'm tracking an event with a good bit of data, and I noticed that it wasn't getting sent to Mixpanel.

First, I made it so verbose=1 could be sent and I still wasn't getting any information back to help track down the cause.

Then, I noticed that in the event of an IOErrorEvent, a '0' is sent to the callback.  I changed that to send the actual event's text.  In my case, it wasn't helpful.  Error #2032: Stream Error.

I then opened up a debugging proxy and noticed I was getting an HTTP Error 414: Request URI too long.

Hence, the change to allow the request method to be set via config.

Example:
 
```actionscript
mixpanel.set_config({"verbose": 1, "request_method": URLRequestMethod.POST});
mixpanel.track("My Event", {propertyName: propertyValue}, myCallback);

private function myCallback(result: String) : void {
   trace("Got this back from Mixpanel: " + result);
}
```