Add `topMargin` to constraint maker (#343)

Source/ConstraintMaker.swift

@@ -93,6 +93,11 @@ public class ConstraintMaker {
         return self.makeExtendableWithAttributes(.rightMargin)
     }
     
+    @available(iOS 8.0, *)
+    public var topMargin: ConstraintMakerExtendable {
+        return self.makeExtendableWithAttributes(.topMargin)
+    }
+    
     @available(iOS 8.0, *)
     public var bottomMargin: ConstraintMakerExtendable {
         return self.makeExtendableWithAttributes(.bottomMargin)