Add BeanQueue

[fangpenlin]

BeanQueue is a lightweight Python task queue framework based on SQLAlchemy, PostgreSQL SKIP LOCKED queries and NOTIFY / LISTEN statements. It makes queue background task with database transaction easier than ever. The tasks can be created like any other ORM objects, added to session and committed altogether with other table rows.

We built this project internally for our own use, and we use it in production already. We think it could be helpful for others, so we open sourced it.

There are actually many PostgreSQL based worker queue system out there. BeanQueue is just one of them. That's why I added a new Work Queues section. To learn more about the alternatives, please see my alternatives section in the readme. I am not sure if I understand those projects enough to write a good short description for them, so I only open an PR for my own project.