listings now initialize with photos as an empty array
adds dotenv and aws-sdk gem
adds photo table and model
AddPhoto use case now does the following:
  -> adds a photo to an s3 bucket
  -> adds the url returned by s3 and the listing id to the photos table
  -> only accepts jpg, jpeg, gif, and png file types
  -> returns an error if an invalid file type is chosen for upload

both in memory and orm database classes have an add photo method
Merge branch 'photos' into development

Conflicts:
	db/schema.rb
	lib/air_cycle/db/orm.rb