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.
You can omit the .js extension in Node require calls and the function will add them automatically.
Some modules like the slice function here perform their work automatically at the time of the require call and don't need to be assigned to anything.