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
Merge pull request #24 from ohwillie/disable-instead-of-stop

Use NodeState#disable instead of NodeState#stop.