Don't include test files in the exported gem

To run a single file, use:

  ruby -I test <filename>

Update background job example

* Per discussion from https://github.com/alexreisner/geocoder/issues/816,
  this commit replaces the `sidekiq_worker` example with an example
  using ActiveJob.
* Demonstrate approach to reverse-geocoding records in a background job
  with simple retry logic for timeout and socket errors.

Enable testing different database environments

Add Geocoder.log()