Convert id to string

[mistermult]

• Add a field object_id of type string, e.g. to track one object across multiple frames (tracking).
• Modify type of (class) id to type string.

See my comment https://github.com/Kukanani/vision_msgs/issues/18#issuecomment-511352609 for the rationale of the design.

[Kukanani]

Updated to only change the type of the id field based on discussion in #18.

Thanks for the PR!