Indexing the guest_token column on spree_orders

![screen shot 2014-11-18 at 10 53 01](https://cloud.githubusercontent.com/assets/3056/5125528/393f017c-70bd-11e4-8f06-3187bf25a267.png)

Adding an index to the guest_token column made a big impact on the performance of the `cart_link` action in our app.