Stamina example application

[agemooij]

We need a proper, fully working standalone example application that shows off a number of migrations and how to test them with the testkit.

The model should be pretty small and open to multiple interpretations so we can grow it from V1 to V5 without it feeling too much like a made-up example. Ideas have included a TODO list app.

It should probably use an embedded leveldb database.

[raboof]

I made https://github.com/raboof/todo-backend-akka previously.

Not particularly proud of it (the API and thus structure is kind of contrived) but it's probably good enough as the basis for a stamina example.