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.
This looks behaving differently.
In my understanding, listObjects will return the list of the all objects with the prefix, so if s3 storage has the following files:
foo/bar.txt
foo/baz/quox.txt
foo/baz/foobar.txt
baz/

and when requested with prefix "foo/", the result is foo/bar.txt, foo/baz/quox.txt, foo/baz/foobar.txt.  And your s3 client parses the result as foo/ has "bar.txt" and a directory "baz".
However, this mock doesn't dig into the directory, so response would be "foo/bar.txt" and "foo/baz/" directory.

This difference would skew the test result.