Kill poller_thread before waiting for it to die

I noticed that recent guard versions sometimes fail to exit if there's a poller thread running: If so, it'll just hang on exit after saying "INFO - Bye bye...". When Ctrl-C'ing that, it will return you to the REPL and quitting immediately afterwards (via "quit") works. 

I tracked this problem down to the listen adapter waiting for a poller thread to quit without actually telling it to terminate.

This patch kills the poller thread before joining the thread, which makes guard reliably exit. I hope you'll consider including this in a release (-: