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.
S3 server groups the result only when Delimiter is set, and your mock only emulates this behavior.
So I am suggesting to check if Delimiter is set to '/' and fails otherwise.  That would not break the test at all because all of your code is currently setting Delimiter correctly, but it seems worth checking so.