Exclude id field from _source

Fixes #78
You're expecting a creation and for no existing annotation with that `id`, so why not use 'create' if that's the expected result?

If we let front-end applications choose the annotation id, the index operation is fine and it will clobber any previous annotation with that `id` but unless we get into serializing the version of the document and sending it back so we can get Check-And-Set semantics we don't even have a good way to distinguish between a creation and an update. `POST` vs `PUT` is not sufficient, IMO. The lack of an `id` means create. The presence means it could be either.

To me, code clarity and simplicity wins out here. I'd kill the argument, I think.