Fix devide by zero bug and properly handle exponential backoff with an alert_after. Closes #2

Primary: @bobtfish  or @hashbrowncipher 

This fixes #2 where we get divide by zero errors.

This was a hard to track down situation that only occurred when we had an alert_after and exponential backoff.

I hope I've made this filtering logic a bit more clear. Luckily we have tests!!! So using this new power-of-two function allowed me to be sure it still is legit.