Fix issue with gevent 1.0 and with ack value with single param

In case the namespace on_XXX method returns a single value, the client library is failing because gevent-socketio is returning a single value and not a list as it should. This short patch fixes this. I also took the workaround to make it work with gevent head.
I have tested the update thanks to the js tests in chrome and firefox, with both latest PiPy versions and latest heads, the tests passed all. I dont have IE so can't tell, but the change should be small enough that it works in there too.