Email producers with order/delivery instructions #275

Would be great to get a review of this as a sanity check.
This will get a flat list of all the line items, but it won't collate them. So we'll get something like this, with lots of repetition:

```
Apples - 5
Beetroot - 7
Carrots - 2
Apples - 2
...
```

We could solve this by aggregating quantities in a hash. Something like:

```ruby
line_items = @orders.map(&:line_items).flatten
line_items.inject({}) do |lis, li|
  lis[li.variant] ||= {line_item: li, quantity: 0}
  lis[li.variant][:quantity] += li.quantity
  lis
end
```

See http://blog.jayfields.com/2008/03/ruby-inject.html for more details on `inject`.

Then in the email template, we could go:

```haml
@line_items.each_pair do |variant, data|
  #{variant.sku} ... #{data[:quantity]} ...
```