Add support for secondary key

[stakats]

Plugins will need to support secondary key for 1.5 upgrade. Waiting for merge of sync code.

[simon]

Dan, should I store only the key, or should I store both the ID and key, and first try retrieving the item by ID, then fall back to retrieving by key? And, in either case, how can I retrieve an item by key?

[dstillman]

(In [2978]) Addresses #971, Add support for secondary key

Zotero.Items.getByKey(key) -- retrieve an Item object by its secondary lookup key

No caching yet -- if this becomes a bottleneck, I'll add some

[dstillman]

Just storing the key should be sufficient. A refresh of a document with ids should replace them with keys.

We'll also eventually need to store server user ids, but that's a separate discussion.

[simon]

(In [3098]) closes #971, Add support for secondary key