minor adjustments: use ROS_INFO(), and publish timestamped messages

[ZeppLu]

Some adjustments I personally consider helpful, but don't know whether they fit everyone, so feel free to reject this PR.

In my case, pros of timestamped message:

• you can measure latency with rostopic delay <topic>
• if image and detection message is published separately, you can match them up by looking into their timestamps

[dusty-nv]

Thanks @ZeppLu, I am currently refactoring the nodes to add support for ROS2 (in addition to retaining support for ROS1). I will merge these changes into ros2 branch, which will then be merged back into master relatively soon.