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.
PEP8 ws.
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__'.