Relay should construct the topic message as a proper JSON string

[seanyen]

Ideally the topic messages received by relay should be a proper JSON string.

For example, if the subscribed topic is /acml_pose, and then the message can be:

"/amcl_pose": {
  "amcl_pose": {
    "header": {
	  "seq": "placeholder",
	  "stamp": "placeholder",
	  "frame_id": "placeholder" 
	},
	"pose": {
	  "pose": {
	    "position": {
		  "x": "placeholder",
		  "y": "placeholder",
		  "z": "placeholder"
		},
        "orientation": {
		  "x": "placeholder",
		  "y": "placeholder",
		  "z": "placeholder",
		  "w": "placeholder"
		}
	  },
	  "covariance": ["placeholder"]
	}
  }
}

Here are existing implementations which does the JSON conversion:

• https://github.com/uos/rospy_message_converter
• https://github.com/RobotWebTools/rosbridge_suite