function DistanceConstraint(bodyA,bodyB,distance,maxForce){
    Constraint.call(this,bodyA,bodyB);

    if(typeof(distance)==="undefined") {
        distance = bodyA.position.distanceTo(bodyB.position);
    }

    if(typeof(maxForce)==="undefined") {
        maxForce = 1e6;
    }

    /**
     * @property {number} distance
     */
    this.distance = distance;

    /**
     * @property {ContactEquation} distanceEquation
     */
    var eq = this.distanceEquation = new ContactEquation(bodyA, bodyB);
    this.equations.push(eq);

    // Make it bidirectional
    eq.minForce = -maxForce;
    eq.maxForce =  maxForce;
}
Constrains two bodies to be at a constant distance from each others center of mass.
@class DistanceConstraint
@constructor
@author schteppe
@param {Body} bodyA
@param {Body} bodyB
@param {Number} [distance] The distance to keep. If undefined, it will be set to the current distance between bodyA and bodyB
@param {Number} [maxForce=1e6]
@extends Constraint