Improve test coverage

[fobo66]

We need to increase amount of tests in project to improve maintainability and ensure that project will be fine in the long run.

Possible action steps:

1. Add more unit tests for classes
2. Cover presenters with tests when possible
3. Record Espresso tests

[fobo66]

During work on #104 some simple tests were added for the core logic. Found out that we mostly don't need to test presenters themselves, instead we need to extract logic from them to interactors and test interactors.