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

chrome/content/zotero/xpcom/data/items.js

@@ -27,6 +27,7 @@
 Zotero.Items = new function() {
 	// Privileged methods
 	this.get = get;
+	this.getByKey = getByKey;
 	this.exist = exist;
 	this.getAll = getAll;
 	this.getUpdated = getUpdated;
@@ -101,6 +102,22 @@ Zotero.Items = new function() {
 	}
 	
 	
+	/**
+	 * Retrieves an item by its secondary lookup key
+	 *
+	 * @param	string	key		Secondary lookup key
+	 * @return	object			Zotero.Item object, or FALSE if not found
+	 */
+	function getByKey(key) {
+		var sql = "SELECT itemID FROM items WHERE key=?";
+		var itemID = Zotero.DB.valueQuery(sql, key);
+		if (!itemID) {
+			return false;
+		}
+		return Zotero.Items.get(itemID);
+	}
+	
+	
 	function exist(itemIDs) {
 		var sql = "SELECT itemID FROM items WHERE itemID IN ("
 			+ itemIDs.map(function () '?').join() + ")";