More data layer changes

- Moved ::_get() and _set() from Collection/Search into DataObject, and
  disabled in Item
- Don't disable new items after save. We now put new objects into the
  DataObjects cache from save() so that changes made post-save are
  picked up by other code using .get().
- Added 'skipCache' save() option to avoid reloading data on new objects
  and adding them to the cache. (This will be used in syncing, where
  objects might be in another library where they're not needed right
  away.) Objects created with this option are instead disabled to
  prevent reuse.
- Modified some tests to try to make sure we're reloading everything properly
  after a save.
- Documented save() options

test/tests/dataObjectUtilitiesTest.js

@@ -13,12 +13,10 @@ describe("Zotero.DataObjectUtilities", function() {
 				yield Zotero.DB.executeTransaction(function* () {
 					var item = new Zotero.Item('book');
 					id1 = yield item.save();
-					item = yield Zotero.Items.getAsync(id1);
 					json1 = yield item.toJSON();
 					
 					var item = new Zotero.Item('book');
 					id2 = yield item.save();
-					item = yield Zotero.Items.getAsync(id2);
 					json2 = yield item.toJSON();
 				});