More examples

[tony]

Carried forward from #42 (comment) by @clokep

• Keep in examples/
• Back via pytest to automatically detect breakages via CI
• Copy-paste friendly
• celery-singleton + celery-brainstorm: How can they operate together? #53: Potentially show interoperation with other base classes

[clokep]

I did some of this, but did not setup automated testing for it. That's a bit harder to do since you have to configure the brokers and backends such. It is probably doable with some effort though.

[tony]

@clokep Awesome!

If I have an idea about the automated testing I will PR it