add support for IotpGateway publish over HTTP

It turns out WIoTP doesn't yet support publishing of Gateway device events or its connected device's events over HTTP. See iot-java/issues/85.

Once it's available update IotpGWConnector to support it (see commented out code) and expose support for it in IotpGateway. Update IotpGWDeviceSample (see commented out code for useHttp).