code: function calcultateDepths(depths, callback) {
      if (depths.parentDepth) {
          // if a depth of the links doesn't exist : assign the parehtDepth +1
          // if not, this link has been already found in the past => don't update its depth
          if (! depths.linkDepth) {
              depths.linkDepth = depths.parentDepth + 1;
          }
      }
      else {
          depths.parentDepth = 0;
          depths.linkDepth = 1;
      }
      callback(null, depths);
  }
docs: Calculate the depth
  
  
  @param a structure containing both url
  {parentUrl : parentUrl, linkUri : linkUri}
  @param callback(error, depth)