Fix for IQKeyboardManager (#53)

Fix to make `CardTextField` selectable via IQKeyboardManager's next and previous buttons.
Resolves #52

Pod/Classes/UI/CardTextField.swift

@@ -506,7 +506,28 @@ public class CardTextField: UITextField, NumberInputTextFieldDelegate {
     }
 
     public override func becomeFirstResponder() -> Bool {
-        // Return false, since this text view is only for background style purposes
-        return false
+        // Return false if any of this text field's subviews is already first responder. 
+        // Otherwise let `numberInputTextField` become the first responder.
+        if [numberInputTextField,monthTextField,yearTextField,cvcTextField]
+            .flatMap({return $0.isFirstResponder()})
+            .reduce(true, combine: {$0 && $1}) {
+            return false
+        }
+        return numberInputTextField.becomeFirstResponder()
+    }
+
+    public override func isFirstResponder() -> Bool {
+        // Return true if any of `self`'s subviews is the current first responder.
+        return [numberInputTextField,monthTextField,yearTextField,cvcTextField]
+        .filter({$0.isFirstResponder()})
+        .isEmpty == false
+    }
+
+    public override func resignFirstResponder() -> Bool {
+        // If any of `self`'s subviews is first responder, resign first responder status.
+        return [numberInputTextField,monthTextField,yearTextField,cvcTextField]
+            .filter({$0.isFirstResponder()})
+            .first?
+            .resignFirstResponder() ?? true
     }
 }