Implement unit tests

[IanMayo]

Once we've got some algorithmic processing in the app we should produce some "demo" unit tests, then follow that model as the app grows.

[IanMayo]

I'm re-opening this. I'd like us to write some model unit tests, that we can use as examples.

Off the top of my head, I can think of these types of unit test:

• any non-ui processing algorithms (these will probably be unique, so there's less value in a model example)
• how we validate each entry type (these will be quite similar)
• how we validate each custom entry form (these will be quite similar)
• reducers? (these will probably be quite similar)