add end pointed connected override to the handler

the new endpoint connected callback is mandatory in the handler
interface, without it the README example doesn't work
only socketio client has endpoints, normal websocket doesn't
Merge pull request #18 from liorsion/master

README fix for endpoint callback
Avoid mSocket NPE if WebSocketClient.disconnect() is called twice
Merge pull request #19 from cpeterso/cpeterso/handle-double-disconnect

Avoid mSocket NPE if WebSocketClient.disconnect() is called twice
Fixed merge conflicts