modularizing server and introduce cli example.

Following up from https://github.com/GoogleCloudPlatform/google-cloud-datastore/pull/50, this sent the server files to a new `/server` directory, as well as introduced a CLI example a little more sophisticated than the one from the earlier PR.
The `cli` example (and future examples) can use these helpers to get a hold of the dataset for more advanced use if necessary, as well as a helper to get a Key without needing to know the hierarchy.