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.
The Delimiter field is only used internally by the actual S3 server. Since we're using WebDAV as the backend, and the only delimiter supported is '/', we can emulate the behaviour without checking the field itself.

I don't think this is a good place to do error checking, such as for missing parameters, since it's only used by the test suite - if a parameter isn't supplied then that's a problem with the test, not the main code.