Let people group transactions by using Orders

You can generate debits in an `Order` with the following:

```ruby
resource = Contribution.last
card_or_bank_account = Balanced::BankAccount.find('/bank_accounts/foobar')
order = Neighborly::Balanced::OrderProxy.new(resource.project)
order.debit_from(source: card_or_bank_account)
```
Not a problem, but it would be nice to have this extracted to other method.