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.
It seems end can be omitted http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35.  This isn't applied to your code, but it's better to support such case.