Allow arbitrator to export accounting data

It would be nice if Arbitrator could automatically store data in an SQLite database so that accounting information can be exported. Since I used GNUcash, it would most excellent if it could be exported to this format. In the future, we'd probably also like to export to some other form of accounting software.

Note that this is an epic task, and it will likely require further analysis before a reasonable estimate can be given.

Start by clarifying the accounting data that Arbitrator should store in SQLite and the GNUcash export format requested in the issue. Define the supported export scope and how future accounting formats would fit; done means the requirements and implementation boundaries are clear enough to estimate.

Written by the indexing model from the issue text.