I see that you are going to have a problem with this project because a human body is not unique. Skin resistance varies from person to person, weather, time of day and other sources of resistive shorts can have the same current profile.
Probably the best thing you can do is detect a current imbalance in the mains. If you bring a pickup loop near a wire caring a current, it will induce a current in the loop. If that same loop is in the field of the return wire as well, there will be no current detected because the equal currents flowing in opposite directions will cancel out. However, if something shorts one side of the line, like say you were to jam a metal coat hanger into one hole of an outlet while standing in a metal bathtub full of water in the rain, then some of the current will be flowing through you instead of returning down the other wire causing a small net current in your detector loop that you can use to trip your device. Sensitivity will be controlled by the size of the pickup loop.
Of course, this is not UL approved and should not be considered as adequate protection of human life.