Make localStorageKey configurable via the Configuration object

Fix for #338. I kept global config, was that supposed to be removed?
this complete file should be rewritten to be similar to https://github.com/quaertym/ember-simple-auth/blob/master/packages/ember-simple-auth-cookie-store/tests/simple-auth-cookie-store/stores/cookie-test.js as it doesn't depend on the global config object anymore.