add proxy support for Request

In China, some mobile operators, such as China Mobile or China Unicom, use their own APN  proxy for mobile network access ( for example, the user of China Mobile with cmwap APN must use proxy 10.0.0.172:80), so I add simple proxy support for http request.
Ref: http://www.cib.com.cn/netbank/en/Electronic_Banking/Mobile_Banking/set.html