Add examples/batch.rb script to demo batch geocoding with the async gem

[sbscully]

• Add examples directory.
• Add batch.rb script that demonstrates batch geocoding using async gem.
• Add async gem to development gem group.

The OpenCage::Geocoder uses open-uri which is a wrapper for Net::HTTP. Net::HTTP works correctly with the async gem, so no library changes are needed.

I benchmarked geocoding 100 items, it took ~55s with one worker and ~5s with 10 workers.