Run handlers in threads

Clockwork currently misses events if you block for too long:

```ruby
# clock.rb

require "clockwork"

include Clockwork

every 1.second, "say.1" do
  puts "Hi! 1 second passed!"
  sleep 10
end

every 3.seconds, "say.3" do
  puts "Hi! 3 seconds passed!"
end
```

This (albeit contrived and fairly extreme) example triggers `say.1` and `say.3` once every 10 seconds. It misses `say.1` 9 times and `say.3` twice. I'm certainly not advocating using the clock as a substitute for workers, but I think there's an argument to be made for handlers that block for a non-trivial amount of time while they schedule potentially large amounts of jobs.