Add Feed and FeedItem

Also:
* _finalizeErase in Zotero.DataObject is now inheritable
* Call _initErase before starting a DB transaction
* removes Zotero.Libraries.add and Zotero.Libraries.remove (doesn't seem like this is used any more)

test/tests/feedItemsTest.js

@@ -0,0 +1,38 @@
+describe("Zotero.FeedItems", function () {
+	let feed;
+	before(function() {
+		feed = new Zotero.Feed({ name: 'foo', url: 'http://' + Zotero.randomString() + '.com' });
+		return feed.saveTx();
+	});
+	after(function() {
+		return feed.eraseTx();
+	});
+	
+	describe("#getIDFromGUID()", function() {
+		it("should return false for non-existent GUID", function* () {
+			let id = yield Zotero.FeedItems.getIDFromGUID(Zotero.randomString());
+			assert.isFalse(id);
+		});
+		it("should return feed item id from GUID", function* () {
+			let feedItem = yield createDataObject('feedItem', { libraryID: feed.libraryID });
+			yield feedItem.forceSaveTx();
+			
+			let id2 = yield Zotero.FeedItems.getIDFromGUID(feedItem.guid);
+			assert.equal(id2, feedItem.id);
+		});
+	});
+	describe("#getAsyncByGUID()", function() {
+		it("should return feed item from GUID", function* () {
+			let guid = Zotero.randomString();
+			let feedItem = yield createDataObject('feedItem', { guid, libraryID: feed.libraryID });
+			yield feedItem.forceSaveTx();
+			
+			let feedItem2 = yield Zotero.FeedItems.getAsyncByGUID(guid);
+			assert.equal(feedItem2.id, feedItem.id);
+		});
+		it("should return false for non-existent GUID", function* () {
+			let feedItem = yield Zotero.FeedItems.getAsyncByGUID(Zotero.randomString());
+			assert.isFalse(feedItem);
+		});
+	});
+});