signalfd support

Was supposed to submit this patch some months ago, but forgot. Redid it today from current kernelsauce/turbo:master.

This adds support for signalfd in the IOLoop.
- Adds add_signal_handler() and remove_signal_handler() to the IOLoop
- Example file that shows usage.

Only supports one handler per signum, which I think is okay. But if you see any use cases for calling multiple handlers per signum, let me know and I'll implement it.