Update tutorial code in ros2 branch and host ros2 tutorials in this repository

There are ROS 1 tutorials hosted on the ROS wiki at http://wiki.ros.org/image_transport/Tutorials, but I don't see any ros2 tutorials hosted anywhere. Also, the tutorial code exists on the `ros2` branch, but it still [includes `ros.h`](https://github.com/ros-perception/image_common/blob/ros2/image_transport/tutorial/src/my_publisher.cpp#L4), so it needs to be ported to ros2 as well.