Add support for contentInset and make ASButtonNode a bit more threadsafe

AsyncDisplayKit/ASButtonNode.h

@@ -37,6 +37,13 @@
  */
 @property (nonatomic, assign) ASVerticalAlignment contentVerticalAlignment;
 
+/**
+ * @discussion insets the title and the image node
+ * 
+ * @param contentEdgeInsets The insets used around the title and image node
+ */
+@property (nonatomic, assign) UIEdgeInsets contentEdgeInsets;
+
 /**
  *  Returns the styled title associated with the specified state.
  *