Allow socks5 support on proxy settings

[shimonoe]

This change allows socks5 on proxy settings by creating an socks5 Dialer on transport layer. Also, this is case insensitive, so user can set proxy by declaring in any case.

[101usb]

@shimonoe , the proxy is not being used If I set a timeout

timeout := 10000 * time.Millisecond
request := gorequest.New().Proxy("socks5://8.8.8.8:9999").Timeout(timeout)
resp,_,err := request.Get("www.google.com").End()
fmt.Println(resp, err)