detect Rails DB connection and use it

This is to make it as easy to use queue_classic with Rails, especially with Rails 4.2 + ActiveJob getting out soon-ish.

This is a work in progress. Any help on getting a test for that working would be more than welcomed as I am minitest noob and don't have much more time today for that.

Thoughts on how to test that with minitest?

Related to: https://github.com/QueueClassic/queue_classic/issues/228

TODO:
- [x] add detection of ActiveRecord and use its connection
- [x] add test
- [x] edit README to reflect this new reality
- [ ] ~~add CHANGELOG entry~~