External httpClient support

[kikimor]

Adds the support to make a Device with a customized http.Client:

dev, err := goonvif.NewDevice(goonvif.DeviceParams{
    Xaddr: "172.22.32.10:1899",
    Login: "username",
    Password: "password",
    HttpClient: &http.Client{Timeout: 1 * time.Second},
})

[crazybber]

thanks!