Fix for issue #294 - Support for UIAppearance in IGListCollectionView…

Summary:
Updated IGListCollectionView:initWithFrame:collectionViewLayout constructor to support UIAppearance for the backgroundColor property.

Fixes #294

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes #298

Differential Revision: D4292394

Pulled By: rnystrom

fbshipit-source-id: a7c4f0e516728b684993f2651eadcc25001de783

Source/IGListCollectionView.m

@@ -13,7 +13,12 @@ @implementation IGListCollectionView
 
 - (instancetype)initWithFrame:(CGRect)frame collectionViewLayout:(UICollectionViewLayout *)layout {
     if (self = [super initWithFrame:frame collectionViewLayout:layout]) {
-        self.backgroundColor = [UIColor whiteColor];
+
+        UIColor *backgroundAppearanceColor = (UIColor *) [[[self class] appearance] backgroundColor];
+        if (!backgroundAppearanceColor) {
+            self.backgroundColor = [UIColor whiteColor];
+        }
+
         self.alwaysBounceVertical = YES;
     }
     return self;