Fixes `AwsS3::listContents()` not returning all contents

Created other pull-request from proper account.
See: http://docs.aws.amazon.com/AmazonS3/latest/dev/ListingObjectKeysUsingPHP.html
Merge branch 'patch-3' of https://github.com/Turneliusz/flysystem into Turneliusz-patch-3
Added tests for #122