Document that app code should not pass 'dataset_id' to 'Key.__init__'.

Only the 'helpers.key_from_protobuf' factory should pass it.

Fixes #293.
Elaborate on restrictions for 'Key._dataset_id'.

Incorporates feedback from @dhermes.
Re-order Connection's RPC methods to match protobuf spec.
Rename 'Connection.rollback_transaction' -> 'rollback'.

Makes it consistent w/ 'commit', as well as matching the protobuf RPC.
Implement 'allocate_ids' RPC on Connection.

Fixes #272.
PEP8 ws.
Note protobuf RPCs mapped by connection methods.

Remove section header comments, per @dhermes feedback.
Merge pull request #299 from tseaver/272-add-allocate_ids-rpc

Fix #272: add 'allocate_ids' RPC to connection
Merge pull request #297 from tseaver/293-clarify_key_datastore_id_parameter

Address #293:  document that app code should not pass 'dataset_id' to 'Key.__init__'.
Refactoring duplicate code between storage.key and storage.bucket.