[WIP] Class-based rewrite

[joostdevries]

This is a proposed rewrite of this module. Basically it does two important things:

• All logic is now class-based with a new, cleaner (IMO) API.
• The worker now executes jobs with the app context.

I've created this rewrite because it drastically reduces the amount of code I have to write to get started and offers a cleaner API in my opinion.

Some stuff I'm still contemplating:

• [ ] A way to make queues configurable in the app config (so you don't have to call create_queue)
• [x] Splitting this PR into two: one for the app-context-based worker and one for the new API (which will require a completely new major version or even different package name). #27

This is still work in progress but any input is greatly appreciated