Exclude id field from _source

Fixes #78
Not sure what you mean by triggering. If an `id` field is present, you would want to allow overwriting as to update an annotation. If there is no `id` field, there's no need to use the `op_type='create'` as nothing can be overwritten. The PUT (update) API by the way already ensures that the `id` field is equal to the one in the URL.

The question is again whether we want to allow the client to choose the annotation id. If we do not, it's all easy and just stripping the id is the trivial solution. Nick's hotfix (1691e2a) strips the `id` and thus disallows choosing it. This makes my `allow_overwrite` addition superfluous, but I would keep it nevertheless, so that if somebody wants to choose the `id`s themselves, it is only a matter of removing `id` from `CREATE_FILTER_FIELDS`.