Merge: lib/popcorn: introduce tasks

This PR is based on #2436

## Popcorn threaded tasks

 Tasks allow you to execute code in another thread than the app listening loop.
 Useful when you want to run some tasks periodically.

 Let's say you want to purge the `downloads/` directory of your app every hour:

~~~nit
 class PurgeTask
	super PopTask

	var dir: String

	redef fun main do
		loop
			dir.rmdir
			3600.sleep
		end
	end
 end

 var app = new App

 # Register a new task
 app.register_task(new PurgeTask("downloads/"))

 # Add your handlers
 # app.use('/', new MyHandler)

 # Run the tasks
 app.run_tasks

 # Start the app
 app.listen("0.0.0.0", 3000)
~~~

Pull-Request: #2437
Reviewed-by: Jean Privat <jean@pryen.org>