Add support for sectionInset in ASCollectionView in property and delegate form

examples/ASCollectionView/Sample/ViewController.m

@@ -95,4 +95,8 @@
   [context completeBatchFetching:YES];
 }
 
+- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section {
+  return UIEdgeInsetsMake(20.0, 20.0, 20.0, 20.0);
+}
+
 @end