added the ability of reversing NumberAndSecondaryStat widgets

I am using this widget and I need to [reverse it](http://www.geckoboard.com/developers/custom-widgets/widget-types/number-and-optional-secondary-stat), which means that if the secondary value is lower, the widget will become red (think of application performances, where today its 100ms and yesterday was 50ms, the widget would show it as an improvement :) ).