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.
I don't think console.error is great for tests.  What about invoking callback with meaningful error message and no data?
Then the test will fail in that case.