Fix disconnect/reconnect issues

Commit 155d78b399 prevented reconnections
from working properly.  This fixes that by creating a new RFB object
after disconnecting or failing.

Furthermore, this ensures that a new connection cannot be opened util
we've actually disconnected (either by timer or by receiving a `close`
event).

Closes #452

include/rfb.js

@@ -482,6 +482,7 @@ var RFB;
                 Util.Debug("Clearing disconnect timer");
                 clearTimeout(this._disconnTimer);
                 this._disconnTimer = null;
+                this._sock.off('close');  // make sure we don't get a double event
             }
 
             switch (state) {