nsIRequestObserver Interface

 Called to signify the beginning of an asynchronous request.

 @param aRequest request being observed
 @param aContext user defined context

 An exception thrown from onStartRequest has the side-effect of
 causing the request to be canceled.

void onStartRequest(in nsIRequest aRequest, in nsISupports aContext)

 Called to signify the end of an asynchronous request.  This
 call is always preceded by a call to onStartRequest.

 @param aRequest request being observed
 @param aContext user defined context
 @param aStatusCode reason for stopping (NS_OK if completed successfully)

 An exception thrown from onStopRequest is generally ignored.

void onStopRequest(in nsIRequest aRequest, in nsISupports aContext, in nsresult aStatusCode)