Add first-pass view model support to collection node. #trivial (#356)

* Add first-pass view model support for collection node. Much more to come!

* Address issues

* Update the gorram license header

* Dear lord

Source/ASTableView.mm

@@ -1629,6 +1629,12 @@ static NSString * const kCellReuseIdentifier = @"_ASTableViewCell";
 
 #pragma mark - ASDataControllerSource
 
+- (id)dataController:(ASDataController *)dataController viewModelForItemAtIndexPath:(NSIndexPath *)indexPath
+{
+  // Not currently supported for tables. Will be added when the collection API stabilizes.
+  return nil;
+}
+
 - (ASCellNodeBlock)dataController:(ASDataController *)dataController nodeBlockAtIndexPath:(NSIndexPath *)indexPath {
   ASCellNodeBlock block = nil;