[] and []= methods were removed.

TimeoutError is deprecated in Ruby 2.3.0. It has been an alias for
Timeout::Error starting in Ruby 1.8.0, and should be safe to replace.

* 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.

Make Geocoder.configure accept a hash (now preferred) and add
Geocoder.config method (for reading config). Deprecate old syntax.