Exclude id field from _source

Fixes #78
Fix (seemingly unrelated) bug in test_annotation

For some reason, by introducing the `'_source':{'excludes':['id']}`,
suddenly Elasticsearch does not store empty objects into arrays in the
`_source` anymore. Elasticsearch bug?
Add test for overwriting other annotations

Demonstrates #82
Allow custom annotation id, but block overwriting

Fixes #82.
Add tests for op_type