[ASCollectionView] Finish support for interoperability with base-class UICollectionViewCells.

This also supports supplementary nodes. It builds off of Adlai's .interop flag but makes necessary
improvements for all of the delegate methods to work in practice with heterogenous cell types.

examples/CustomCollectionView/Sample/ImageCollectionViewCell.m

@@ -0,0 +1,46 @@
+//
+//  ImageCollectionViewCell.m
+//  Sample
+//
+//  Created by Hannah Troisi on 1/28/17.
+//  Copyright © 2017 Facebook. All rights reserved.
+//
+
+#import "ImageCollectionViewCell.h"
+
+@implementation ImageCollectionViewCell
+{
+  UILabel *_title;
+  UILabel *_description;
+}
+
+- (id)initWithFrame:(CGRect)aRect
+{
+  self = [super initWithFrame:aRect];
+  if (self) {
+    _title = [[UILabel alloc] init];
+    _title.text = @"UICollectionViewCell";
+    [self.contentView addSubview:_title];
+    
+    _description = [[UILabel alloc] init];
+    _description.text = @"description for cell";
+    [self.contentView addSubview:_description];
+    
+    self.contentView.backgroundColor = [UIColor orangeColor];
+  }
+  return self;
+}
+
+- (void)layoutSubviews
+{
+  [super layoutSubviews];
+  
+  [_title sizeToFit];
+  [_description sizeToFit];
+  
+  CGRect frame = _title.frame;
+  frame.origin.y = _title.frame.size.height;
+  _description.frame = frame;
+}
+
+@end