Add the NestedSort extension: stable sorting for Grid instances.

Nested grid sorting was briefly mentioned in [this pull request](https://github.com/SitePen/dgrid/pull/35). This was a feature we needed internally so we've gone ahead and developed the extension.

The dgrid/extensions/NestedSort module provides stable sorting for Grid instances. It overrides the _constructSort() method, which is in charge of taking a targeted column that has presumably been clicked on and returning an array to set as the active sort. I've included intern tests for the extension.

Since this is my first non-trivial request, I've submitted an Individual CLA to The Dojo Foundation. A Corporate CLA for RGM Advisors LLC will be submitted soon to cover my individual one.

Please let me know if you would like anything else added to the branch.