Fixes to Agreement Search Transaction

- Added searchTransactions method to include params
- Updated Tests accordingly.
- Updated Samples accordingly

sample/billing/SearchBillingTransactions.php

@@ -0,0 +1,24 @@
+<?php
+
+// # Search Billing Transactions Sample
+//
+// This sample code demonstrate how you can search all billing transactions, as documented here at:
+// https://developer.paypal.com/webapps/developer/docs/api/#search-for-transactions
+// API used: GET /v1/payments/billing-agreements/<Agreement-Id>/transactions? start-date=yyyy-mm-dd&end-date=yyyy-mm-dd
+
+// Retrieving the Agreement object from Create Agreement From Credit Card Sample
+/** @var Agreement $agreement */
+$agreement = require 'GetBillingAgreement.php';
+$agreementId = $agreement->getId();
+use PayPal\Api\Agreement;
+
+try {
+    $result = Agreement::searchTransactions($agreementId,array('start_date' => '2013-01-01', 'end_date' => '2015-01-20'), $apiContext);
+} catch (Exception $ex) {
+    ResultPrinter::printError("Search for Transactions", "AgreementTransaction", $agreementId, null, $ex);
+    exit(1);
+}
+
+ResultPrinter::printResult("Search for Transactions", "AgreementTransaction", $agreementId, null, $result);
+
+return $agreement;