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.
We can always refactor this later with a utility class to handle the duplicate code, or maybe move it into one of the NInject modules if we keep that code.