Sanitize Input for Price Variables

- Updated the model to automatically format the price
- Updated the samples to reflect the new changes
- More Unit Tests

sample/sale/RefundSale.php

@@ -20,7 +20,7 @@ use PayPal\Api\Sale;
 // field to mention fees refund details.
 $amt = new Amount();
 $amt->setCurrency('USD')
-    ->setTotal('0.01');
+    ->setTotal(0.01);
 
 // ### Refund object
 $refund = new Refund();