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.
I did some refactoring in the `server/` files. Nothing should have changed any behavior, but if you see anything alarming, I'll be happy to fix it.