Alipay

- Add `Alipay::Mobile::Service.mobile_security_pay_string` method, thanks Sen 49
- Remove `Alipay::Service.mobile_security_pay_url` 49

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- add `refund_fastpay_by_platform_pwd` method
https://github.com/lxneng/alipay/pull/26

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- replace open() calls with io.open() for Python 3 compatibility,
fix `UnicodeDecodeError`
- add `create_direct_pay_by_user_url` doc for Wap site

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- add `single_trade_query` method
https://github.com/lxneng/alipay/pull/20

- Don't warning when `rmb_fee` is used in forex_refund_url method.

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- add `create_forex_trade_url` method
- add `create_forex_trade_wap_url` method
- add `create_batch_trans_notify_url` method