Make 'tox -e docs' rebulid docs from scratch.

Avoid make-like optimizations which can get out of sync after git operations.
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.
Merge pull request #283 from tseaver/tox_rebuild_docs_from_scratch

Make 'tox -e docs' rebulid docs from scratch.
Drop 'Dataset.query' and 'Dataset.entity' convenience APIs.

Point instead to constructing Query and Entity directly.

Addresses #260, but only partially, for gcloud.datastore.
Add a factory registry to datastore._helpers.

Work toward breaking cycles based on import-for-construction.
Break import cycles via a factory registry.

Note that some classes have more than one factory (class methodsd like
'from_protobuf' and 'from_path').  Each is registered under a separate name.

Addresses #260 (for gcloud.datastore).
Tidy local imports, remove remaining cycles.
Fix build:  invoke correct factory when unmarshalling key from pb.
Reorder 'Query.__init__' paramters to match Entity.
Rename '_query_factory' helper -> '_get_factory' to avoid confustion.

Incorporates feedback from @dhermees.
Avoid relative imports.

Incorporate feedback from @dhermes.
Encapsulate factory registry as a singleton object.

Use its methods, rather than free functions.
Rename alternate factories for clarity.
Use 'invoke' rather than 'get' followed by call to invoke factories.