Fix #272: add 'allocate_ids' RPC to connection

To ease comparison with the protobuf spec:

- Reorders other RPC methods to match the order in the protobuf spec
- Renames 'Connection.rollback_transaction' -> 'rollback'.
Doesn't need to be changed, just wanted to point out.

The phrase "dataset in the Cloud Datastore" makes it seem like there is just one cloud datastore where all the things are, which is I suppose true in many ways (i.e. on service) and untrue in others.