Allow the setting of fixed items in localStorage

I'd like to be able to add items which aren't cleared out if there is no room in `localStorage`.

If my FIFO queue were full and I were to call `localStorage.setItem(key, value)` then my call would fail. However by using the `setFixed(key, value, removed)` method on __fifo__ then I can safely add fixed items.

Tests also included.

Also closes #1