Update statement generation

It should generate statements that can be displayed like this:
![statement](https://cloud.githubusercontent.com/assets/1731223/5044294/c63c2cd2-6bf7-11e4-9b84-8f98883fe53b.png)




What we used to do is to pass the ``account.id`` and then retrieve the account object from Django inside the task (and get the ``account_number`` from that).