allow multiple authors to edit a single hyper reading list.

[e-e-e]

This functionality is already available in hyperdb, and just needs to be exposed via hyperreadings.

This will also open up the huge can of worms around how to deal with conflicts.

[e-e-e]

We could do this now - but I want to wait until we have version controlling #40. Because the db is built as an indexed triple store but hyperdb is version per insert of key, there are moments in the hyperdb history where a hyper-reading is not fully constructed object - and as such could break.

If we add versioning in a way where each time someone saves a version is tagged, we can provide users with a clearer means of comparing conflicts - eg, take their version or your version. It will also be clearer when conflicts may have occurred as we can tell if another writer has saved a version while you were editing the document.