code: function(o1, o2, key)
  {
     var rval = 0;
     if(key in o1)
     {
        if(key in o2)
        {
           rval = _compare(o1[key], o2[key]);
        }
        else
        {
           rval = -1;
        }
     }
     else if(key in o2)
     {
        rval = 1;
     }
     return rval;
  }
docs: Compares two keys in an object. If the key exists in one object
  and not the other, the object with the key is less. If the key exists in
  both objects, then the one with the lesser value is less.
  
  @param o1 the first object.
  @param o2 the second object.
  @param key the key.
  
  @return -1 if o1 < o2, 0 if o1 == o2, 1 if o1 > o2.