Use NodeState#disable instead of NodeState#stop.

NodeState#disable truly turns off the FSM. It calls NodeState#stop internally, 
but it sets the disabled flag as well, which is checked inside wrapped
callbacks and before any transitions.

https://github.com/ichernev/node-state/blob/master/lib/nodestate.js#L127 
https://github.com/ichernev/node-state/blob/master/lib/nodestate.js#L62 
https://github.com/ichernev/node-state/blob/master/lib/nodestate.js#L150