Comparison function for naturally sorting strings

Jeff Atwood [says it best](http://blog.codinghorror.com/sorting-for-humans-natural-sort-order/):

> I can say without the slightest hint of exaggeration that this exact sorting problem has been a sore point on every single project I've ever worked on. Users will inevitably complain that their items aren't sorting properly, and file bugs on these "errors".
