Fix Livy Batch model and tests, resolve Issue #56

[zeninpalm]

Hi, this PR resolves the issue #56 by fixing the Livy Batch model and related tests.

The current Batch model is based on the official documentation from Livy at https://livy.incubator.apache.org/docs/latest/rest-api.html, which unfortunately is not inline with the current code of the Livy server.

The current code of the Livy server already returns "name", "owner" and "proxyUser" information for a batch, but the official documentation is not updated yet. So I patched our current code to reflect the latest changes.