New Admin Order Interface (powered by Ember)

I've spent a large portion of the large 2-3 weeks working on ripping out the code for Spree's backend order interface and replacing it with something built with Ember. It is now *mostly* complete, but as we all know the last 20% of the work takes 80% of the time. 

What still needs doing:

- [ ] Sanity checking of the code by an experienced Ember developer
- [ ] Shipment splitting
- [ ] Creating a new payment
- [ ] Capturing payments
- [ ] Voiding payments
- [ ] Refunding a payment
- [ ] Adding a new adjustment
- [ ] Tests, using the current Capybara tests as a base