Use thread for PubsubhubbubNotifier#notify

If the method blocks, it keep user waiting until post is over.
It slows response time especially when posting to many hubs.
Joining to main thread is not needed because user and application
need not know whether posting is succeeded or not. We can see it by log.
Merge pull request #109 from iiirc/pubsubhubbub-thread

Use thread for PubsubhubbubNotifier#notify