Multiple-attribute sorting

This pull request extends the functionality of the `_sort()` method so that it can accept a comma-delimited string and sort down to a depth of n. Here's a [working demo](http://codepen.io/ultimatedelman/pen/KeEpl) that sorts down to 3 levels.

The codebase is at the top, the demo code to make it work is at the very bottom of the JS.