Uniq test for unequivalent object literals

The exact implementation of the _.uniq function may be unclear to people wishing to improve the algorithm.  
Rather than add extra lines of comments to the underscore.js file, a simple test should make the input/output expectations for _.uniq crystal clear.

```sh
var a = {}, b = {}, c = {};
deepEqual(_.uniq([a, b, a, b, c]), [a, b, c], 'works on values that can be tested for equivalency but not ordered');
```