Implementing remaining datastore regression tests from gcloud-node.

@tseaver Thanks for finishing `allocate_ids`, this wouldn't have been possible without it. The `TestDatastoreAllocateIDs` test is a bit clunky but as @jgeewax mentioned today, implementing part of the API surface to allocate IDs without pbs may not be an M1 priority.

@silvolu I think I've grokked what is happening in the [transaction][1] test (in `gcloud-node`) but am not 100%. Can you double check `TestDatastoreTransaction`?

[1]: https://github.com/GoogleCloudPlatform/gcloud-node/blob/51abc0dbd59139aa7d44020f5fb5291e56ae660e/regression/datastore.js#L384