feat: make Scheduler thread-safe

schedule() can now be called from any thread, but will move tasks to the event loop
thread. pthread_kill(main_thread, SIGIO) is used to ensure prompt reponse if needed.

This enables receiving I/O notification from "sidecar runtimes" like async-compat, for
instance.

The async example has been rewritten to use async_::spawn, demonstrating usage of
reqwest and hyper clients wrapped in Compat to provide a tokio runtime environment while
using the async_ Scheduler as executor.

Author: pschyska