Async callbacks

[kato-m]

Hi authors,

great package! Exactly what we would need, except that my callbacks would be async. Have you already thought about supporting async callbacks, or are there technical reasons why you haven't done it?

I haven't looked at it in details, but depending on the effort, we could think about implementing an async engine. What is your opinion on how complex it would be? Any pitfalls you could think of?

Any advice would be very m,uch appreciated. Thanks a lot!