datastore support set null value

[lucemia]

based on cloud datastore protobuf, it is possible to set null to an attribute.
currently gcloud will raise exception raise ValueError("Unknown protobuf attr type %s" % type(val)) while user trying to set null for any attribute.

// A message that can hold any of the supported value types and associated
// metadata.
//
// At most one of the <type>Value fields may be set.
// If none are set the value is "null".
//
message Value {
// ...
}

https://github.com/GoogleCloudPlatform/gcloud-python/blob/b6d3e74a48e8554804ea3d33f53385bbbdb5c4b7/gcloud/datastore/helpers.py#L135

[tseaver]

Via #296.