Support Payment sandbox mode

[skyred]

Following discussion from #507

This PR adds the feature. So, app user can set sandbox mode like this:

$app = new Application($options);
$wechat_pay = $app->payment;
$wechat_pay->enableSandbox(TRUE);

Another option to enable to sandbox can be in $option:

    $options = [
      // 前面的appid什么的也得保留哦
      'app_id' => $appid,
      // ...
      // payment
      'payment' => [
        'merchant_id'        => $mch_id,
        'key'                => $key,
        'cert_path'          => 'path/to/your/cert.pem', // XXX: 绝对路径！！！！
        'key_path'           => 'path/to/your/key',      // XXX: 绝对路径！！！！
        'notify_url'         => $payment_gateway_plugin->getNotifyUrl()->toString(),
         'device_info'     => '013467007045764',
         'sub_app_id'      => '',
         'sub_merchant_id' => '',
         'sandbox_mode' => TRUE,
        // ...
      ],
    ];

But, this PR does't include the second way. Further contribution is welcomed.

[overtrue]

TRUE -> true

[overtrue]

- if ($isSandboxMode == TRUE) {
+ if ($isSandboxMode) {

[overtrue]

- protected $sandboxMode = '';
+ protected $prefix = '';

[skyred]

It looks like PaymentAPITest.php has failed. https://travis-ci.org/overtrue/wechat/jobs/224199863#L342

[overtrue]

[overtrue]

请在本地测试通过再提交，谢谢。

[skyred]

Let me close this PR, and do a clean one.

[mingyoung]

@skyred 沙箱模式的签名是不是要从微信服务器获取？