Provide a 'tombstone' option to indicate that Bag was deleted

[mjordan]

If a bag has been deleted, we should not delete its document in the index. Rather, we should

1. update the tombstone field from false to 'true and
2. update the document's timestamp

That way, the last state of the document, and all of its previous versions, remain in the index.

[mjordan]

Since the watcher script will provide the path of the deleted bag as the input to the tombstone script, we will need to resolve #11 before this issue can be closed.

[mjordan]

Fixed with a30ce7e.