Reduced redundant code.

php.net\Exception has an exception code.
TaxJar\Exception adds an exception code, blocking usage of the php's exception code, and php's previous.

lib/Exception.php

@@ -3,16 +3,11 @@
 
 class Exception extends \Exception
 {
-    protected $status_code;
-
-    public function __construct($message, $status_code = 0)
-    {
-        $this->status_code = $status_code;
-        parent::__construct($message);
-    }
-
+    /**
+     * Alias to getCode, for backwards compatibility
+     */
     final public function getStatusCode()
     {
-        return $this->status_code;
+        return $this->getCode();
     }
 }