added get account balance

src/OldTimeGuitarGuy/MechanicalTurk/Operations/GetAccountBalance.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace OldTimeGuitarGuy\MechanicalTurk\Operations;
+
+/**
+ * The GetAccountBalance operation retrieves the amount of money in your Amazon Mechanical Turk account.
+ *
+ * http://docs.aws.amazon.com/AWSMechTurk/latest/AWSMturkAPI/ApiReference_GetAccountBalanceOperation.html
+ */
+class GetAccountBalance extends Base\Operation
+{
+    /**
+     * Determine if the given parameters satisfy this operation's requirements
+     *
+     * @param  array  $parameters
+     *
+     * @return boolean
+     */
+    protected function satisfiesRequirements(array $parameters)
+    {
+        return true;
+    }
+}

src/OldTimeGuitarGuy/MechanicalTurk/Requester.php

@@ -26,6 +26,7 @@ class Requester implements Contracts\Requester
         'registerHITType' => Operations\RegisterHITType::class,
         'rejectAssignment' => Operations\RejectAssignment::class,
         'approveAssignment' => Operations\ApproveAssignment::class,
+        'getAccountBalance' => Operations\GetAccountBalance::class,
         'getReviewableHITs' => Operations\GetReviewableHITs::class,
         'setHITAsReviewing' => Operations\SetHITAsReviewing::class,
         'getAssignmentsForHIT' => Operations\GetAssignmentsForHIT::class,