Added support of xoffset and yoffset to the moveto command

Sample usage:

        $this->moveto(array(
            'element' => $this->byId('moveto'),
            'xoffset' => 10,
            'yoffset' => 10,
        ));

or

        $this->moveto(array(
            'xoffset' => $deltaX,
            'yoffset' => $deltaY,
        ));

Protocol reference: https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/moveto