Null references when Agent and Batch are over.
Rename Agent.unload to Agent.destroy.
setExecuting(false) when test skipped due to 404.
Update code comments.

 - GC is done now.
 - SockJS replaced Socket.io.
Null refs on complete. If browsers AWOL, complete.
Set destroyed bit when Agent is destroyed.
Skip health check if agent was destroyed.

Agent can be destroyed externally by a WebDriverCollection.
Check didComplete before calling next.
Check if destroyed before queueing messages.
Destroy Batch properties after browsers close.
Check if destroyed before handling close event.
Do not handle agentEmitter events if destroyed.
Fix linter errors.
Return early in Agent#nextURL if destroyed.
Test Node 0.10 in Travis CI.
Merge branch 'hub-memleak'