Use WritableSettingsStore

# Work in Progress

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'm not sure how I feel about duplicating this `[TestFixtureSetUp]` block just to load the MEF components properly, but I'm not sure we can avoid it either.