Add class to mock the AWS JS SDK for unit testing

This PR deals with google/chromeos-filesystems#5.

Not finished yet, just starting a PR to kick off discussion.

## Todo:
- [x] Implement the `getObject` response body class with its `toArrayBuffer` method.
No, I think `listObjects` returns only files that are immediate children of the prefix/directory. Any sub prefixes/directories are returned in the `CommonPrefixes` list, which you can see below.