datastore.get should handle entity not found better

Currently datastore.get would return `callback(null #error, undefined #key, undefined #entity), when entity is not found.

I think it would be more useful for the developer to provide a more explicit hint that the entity was not found (either through a dedicated error or some more structured result value).

As a reference the underlying RPC returns found and missing arrays.

I was thinking that current case (null #error, undefined #key, undefined #entity) would be explicit enough to say entity is not found. Returning a dedicated error would be another option but it gets annoying to handle errors on methods that work with multiple entities (getAll, saveAll, deleteAll).

maybe cb(null, null, null) would be more appropriate that cb(null, undefined, undefined).

I'm not sure what nodejs developer expect for empty result, what seems weird in my case is that I can undefined as the result even if my lookup RPC is valid.

I will pass null, null, null for consistency. Developer's way of interacting with this library is likely to be changed -- depending on which subset of JavaScript they like for sure.