Allow order admin search via sku

This PR allows admins to search for orders based on SKUs in the order. This is potentially a performance hit if there are thousands of SKUs when populating the select widget, but I imagine the same problem exists for the promotions filter drop down as well.

![screen shot 2014-10-16 at 4 22 49 pm](https://cloud.githubusercontent.com/assets/134368/4669962/b39b2286-5572-11e4-9d78-83a0fe183262.png)