Varun/checkout page

This PR implements checking out.

A partner can add books to their cart and push the order to the worldreader admins for review.

Fixes #94
Fixes #121 
Fixes #122 
Might be a little cleaner to use member/collection routes

```ruby
resources :carts do
  member do
    post :remove
  end

  collection do
    post :create_purchase
  end
end
```

Should double check routes that need to get updated though.