Use native layout margins when available

This makes `preservesSuperviewLayoutMargins` work properly on supported
systems (iOS 8+).

Pod/Classes/OAStackView.m

@@ -22,6 +22,9 @@ @interface OAStackView ()
 
 // Not implemented but needed for backward compatibility with UIStackView
 @property(nonatomic,getter=isBaselineRelativeArrangement) BOOL baselineRelativeArrangement;
+
+@property(nonatomic, getter=areNativeLayoutMarginsSupported, readonly) BOOL nativeLayoutMarginsSupported;
+
 @end
 
 @implementation OAStackView
@@ -69,14 +72,16 @@ - (instancetype)initWithFrame:(CGRect)frame {
 }
 
 - (void)commonInitWithInitalSubviews:(NSArray *)initialSubviews {
+  _nativeLayoutMarginsSupported = [UIView instancesRespondToSelector:@selector(layoutMargins)];
+
   _mutableArrangedSubviews = [initialSubviews mutableCopy];
   [self addViewsAsSubviews:initialSubviews];
 
   _axis = UILayoutConstraintAxisHorizontal;
   _alignment = OAStackViewAlignmentFill;
   _distribution = OAStackViewDistributionFill;
 
-  _layoutMargins = UIEdgeInsetsMake(0, 8, 0, 8);
+  _compatibilityLayoutMargins = UIEdgeInsetsMake(0, 8, 0, 8);
   _layoutMarginsRelativeArrangement = NO;
 
   self.alignmentStrategy = [OAStackViewAlignmentStrategy strategyWithStackView:self];
@@ -211,9 +216,23 @@ - (void)setDistributionValue:(NSInteger)distributionValue {
   self.distribution = distributionValue;
 }
 
+@synthesize layoutMargins = _compatibilityLayoutMargins;
+
+- (UIEdgeInsets)layoutMargins {
+  if (self.nativeLayoutMarginsSupported) {
+    return super.layoutMargins;
+  }
+
+  return _compatibilityLayoutMargins;
+}
+
 - (void)setLayoutMargins:(UIEdgeInsets)layoutMargins {
-  _layoutMargins = layoutMargins;
-  [self layoutArrangedViews];
+  if (self.nativeLayoutMarginsSupported) {
+    [super setLayoutMargins:layoutMargins];
+  } else {
+    _compatibilityLayoutMargins = layoutMargins;
+    [self layoutArrangedViews];
+  }
 }
 
 - (void)setLayoutMarginsRelativeArrangement:(BOOL)layoutMarginsRelativeArrangement {
@@ -227,6 +246,11 @@ - (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
   [self layoutArrangedViews];
 }
 
+- (void)layoutMarginsDidChange {
+  [super layoutMarginsDidChange];
+  [self layoutArrangedViews];
+}
+
 #pragma mark - Adding and removing
 
 - (void)addArrangedSubview:(UIView *)view {