Use WritableSettingsStore

Fixes #70

This pull request updates the code to use the `WritableSettingsStore` instead of the application settings mechanism to store cached credentials. I am still a bit uncomfortable with certain aspects of the new implementation.

I'm creating the pull request early to allow more time for review and comments.
Actually, after looking at it a bit more it's become clear that `[SetUp]` is what we want. Otherwise, the behavior of the tests becomes order-dependent.