Handle PEP 475 fallout

Closes #255 (closed)

As of Python 3.5, PEP 475 gets in our way. Runners with long time.sleep()'s in their _snooze() method (e.g. the retry runner) will have their system call implemented time.sleep() automatically retried at the C layer. The only reliable way to prevent this is to raise an exception in the signal handler. The standard run() method automatically suppresses this exception, meaning, it's caught and ignored, but effectively breaks the run() loop, which is just what we want.

The lmtp and rest runners implement their own run loops, so they also have to handle this exception, by ignoring it.