added transaction fee support to sale class

lib/PayPal/Api/TransactionFee.php

@@ -0,0 +1,70 @@
+<?php
+
+namespace PayPal\Api;
+
+use PayPal\Common\PayPalModel;
+use PayPal\Converter\FormatConverter;
+use PayPal\Validation\NumericValidator;
+
+/**
+ * Class TranactionFee
+ *
+ * Let's you specify details of the transaction fee.
+ *
+ * @package PayPal\Api
+ *
+ * @property string currency
+ * @property string value
+ */
+class TransactionFee extends PayPalModel
+{
+    /**
+     * 3 letter currency code
+     *
+     *
+     * @param string $currency
+     *
+     * @return $this
+     */
+    public function setCurrency($currency)
+    {
+        $this->currency = $currency;
+        return $this;
+    }
+
+    /**
+     * 3 letter currency code
+     *
+     * @return string
+     */
+    public function getCurrency()
+    {
+        return $this->currency;
+    }
+
+    /**
+     * Total amount charged from the Payer account (or card) to Payee. In case of a refund, this is the refunded amount to the original Payer from Payee account.
+     *
+     *
+     * @param string|double $value
+     *
+     * @return $this
+     */
+    public function setValue($value)
+    {
+        NumericValidator::validate($value, "value");
+        $value = FormatConverter::formatToPrice($value, $this->getCurrency());
+        $this->value = $value;
+        return $this;
+    }
+
+    /**
+     * Value amount charged from the Payer account (or card) to Payee. In case of a refund, this is the refunded amount to the original Payer from Payee account.
+     *
+     * @return string
+     */
+    public function getValue()
+    {
+        return $this->value;
+    }
+}