Add option for single-shot detection and corresponding service.

[neomanic]

We recently decided to switch from continuous tag detection to upon request, and I was surprised to see that the single image node actually reads and writes files from disk. The way I expected it to work was a service call to trigger a tag detection.

I've implemented a quick version of what I expected here. It extends the ContinuousDetector with a single_shot_detection parameter (default: false), which went set to true only runs tag detection after a service call. The service handler subscribes to the camera topic, and then only allows one image through before unsubscribing.

Comments and changes welcome.