Sanitize Input for Price Variables

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

lib/PayPal/Validation/NumericValidator.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace PayPal\Validation;
+
+/**
+ * Class NumericValidator
+ *
+ * @package PayPal\Validation
+ */
+class NumericValidator
+{
+
+    /**
+     * Helper method for validating an argument if it is numeric
+     *
+     * @param mixed     $argument
+     * @param string|null $argumentName
+     * @return bool
+     */
+    public static function validate($argument, $argumentName = null)
+    {
+        if (trim($argument) != null && !is_numeric($argument)) {
+            throw new \InvalidArgumentException("$argumentName is not a valid numeric value");
+
+        }
+        return true;
+    }
+}