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
exclude demo from test coverage report
Merge pull request #234 from silvolu/exclude-demo-coverage

exclude demo from test coverage report
Remove TODO from gcloud.storage.bucket.

Re-introduced in 2f930652860400831e9fc1f3ef2dbf7179fcce26 after
being removed for #162 as part of the effort in #175.
Updated docstring on Key.rename() with a warning.

I have a pending request to the GCS team asking if this is a reasonable thing to have exposed, however based on our conversations in #99 and #224 it seems that it needs to exist regardless (if we don't provide it, people will just do this on their own).

That said, we need to be clear about what's happening under the hood in the short part of the docstring. If someone is frequently "renaming" 5TB objects (that's the maximum size currently), this could actually change their bill a lot and make the GCS API do some ridiculous amounts of work. For small objects this shouldn't really be a problem.
Revert "Updated docstring on Key.rename() with a warning."

This reverts commit aa2e66a7de78d46aa4b9b7bc02ba6acb84406ae5.

This was meant as a pull-request, not a direct commit.
Finish un-API-ifying datastore._helpers by removing from docs.