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 am not saying this doesn't work.
I am saying, you should put the require to the place which actually requires it.  It seems helper.js doesn't use slice directly but s3mock.js does, so it's better to put the require() there.

Sometimes you want to import s3mock.js in another file, in that case, keeping the dependency from s3mock inside of s3mock would simplify the code.