Add debug property support for STOMP over websocket notifier

The debug property support is missing. Without the debug property support it is hard to find out the data being sent and received from the stomp library.

As mentioned in http://jmesnil.net/stomp-websocket/doc/ the following property can be used for debugging:

client.debug = function(str)

{ // append the debug log to a #debug div somewhere in the page using}

;