Added config option to select Websocket protocol (ws:// or wss://)

Hi there,

We at the Federal University of Santa Catarina were having some problems with websocket connections from our clients: although we use https, the websocket connections were being made using ws:// instead of wss://.
Investigating the issue, we verified that the sole use of $isHTTPS in the "vpl_submission_CE.class.php" file was not enough to correctly determine the type of websocket protocol to use in our case (our servers are behind a proxy; the connection from the clients to the proxy are made using https, and the connections behind the proxy use http).

We created an additional option to select which protocol must be used to solve our problem. If none is selected, the default behavior is to consider $isHTTPS.

Thanks!