Sanitize Input for Price Variables

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

tests/PayPal/Test/Api/CurrencyTest.php

@@ -18,7 +18,7 @@ class CurrencyTest extends \PHPUnit_Framework_TestCase
      */
     public static function getJson()
     {
-        return '{"currency":"TestSample","value":"TestSample"}';
+        return '{"currency":"TestSample","value":"12.34"}';
     }
 
     /**