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.
I think this is my mistake, it is just more obvious with your code added. I think your code and the Rebinds should probably be in `[TestFixtureSetUp]` and the last line should probably stay in `[SetUp]`