Added "read" method to S3Client and FileClient classes. This wa, ImportSubscriptionsJob reads the OPML file using this method, instead of trying to read dire
ctly from the filesystem. This means that in production and staging environments ImportSubscriptionsJob reads the OPML file from Amazon S3, which is the expe
cted behavior.

Also changed unit tests for ImportSubscriptionsJob and SubscriptionsImporter classes, to check that the right Feedbunch::Application.config.uploads_manager m
ethods are called, instead of assuming filesystem access.

Also added a skeleton aws_key.rb file (without the actual AWS keys) to github. This file is necessary during development and testing (including testing in the CI environment) because it requires the aws-sdk gem and performs some initialization.