nsIStreamLoader Interface

 Asynchronously loads a channel into a memory buffer.

 XXX define behaviour for sizes >4 GB


 Initialize this stream loader, and start loading the data.

 @param aChannel
        A Channel to load data from. This must not be asyncOpen'd yet!
 @param aObserver
        An observer that will be notified when the data is complete.
 @param aContext
        May be null. Will be passed to the observer.

 @note Failure to open the channel will be indicated by an async callback
       to the observer.

void init(in nsIChannel aChannel, in nsIStreamLoaderObserver aObserver, in nsISupports aContext)