Add explicit support for 'entity_value' in protobufs.
Lint removal.
Add helpers.set_protobuf_value.

Handles compount fields (entity, key) not assignable via setattr.
Use helpers.set_protobuf_value to support composite types.
Fix docstring param name.

Incorporates feedback from @dhermes.
Drop XXX comment.

Entities are not required to have datasets w/ active connections.
Don't set default key on entity protobuf if entity has no key.

Incorporates feedback from @dhermes.
Rename helper functions with leading underscore.

Signal that they are *not* part of the gcloud-python API.
Update doctring to indicate that helper functions are not API.

Note that these are functions, not methods.
Rename datastore.helpers -> _helpers.

Further emphasize non-API nature.
Merge pull request #232 from tseaver/support-entity-protobufs

Support entity protobufs