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.
So what, just `console.error` a message and return?

I can't pass an error to the callback because that's not consistent with the S3 API I'm mocking