*
@method contains(<d>)
@returns Boolean
@short Returns true if <d> is contained inside the DateRange. <d> may be a date or another DateRange.
@example

Date.range('2003', '2005').contains(Date.create('2004')) -> true

*
function(obj) {
      var self = this, arr = obj.start && obj.end ? [obj.start, obj.end] : [obj];
      return arr.every(function(d) {
        return d >= self.start && d <= self.end;
      });
    }