Don't copy key's datset ID to the protobuf.

The back-end already knows it, and it is actually different than the
'project ID' we are assigning to the dataset as its ID.

Fixes #121.
Remove direct 'Key' -> 'Dataset' relationship.

Replace it with an (optional) '_dataset_id' attribute, which is None for
newly-created keys, or the more-or-less opaque value set by the backend
for keys fetched from it.

Add direct 'Entity' -> 'Dataset' relationship (rather than indirecting
through the entity's key.

Addresses @pcostell's remarks on #259:

https://github.com/GoogleCloudPlatform/gcloud-python/pull/259#issuecomment-59971468
Merge pull request #276 from tseaver/121-dont_copy_key_datasetid_to_pb

Fix #121: don't copy project ID to the Key protobuf's dataset_id.