Add "Canceled" shipment state.

![screen shot 2014-03-14 at 9 49 35 pm](https://f.cloud.github.com/assets/193112/2421253/93a4420c-ab7f-11e3-8ea0-c35ef53a97ff.png)

Should we...

- Add a new "Canceled" shipment state.
- Remove the old "Dispatched" state, it is not being use in anywhere.

Reference: http://guides.spreecommerce.com/developer/shipments.html