Remove token and user id keys from session and local storage on logout

Remove 'accessTokenId' and 'currentUserId' keys from session and local storage on logout.
Fixes #61 
I'm very new to the community contribution paradigm and your guidance here is highly appreciated. The main point here is not whether we set the storage key to some value or remove it all together, but make sure it is done in both sessionStorage and localStorage on logout. So I followed your advice and used ```save(storage, name, null)``` instead of introducing a new ```remove``` function. 

Having done that I think removing keys altogether on logout is a cleaner approach for 2 reasons:

1. (Technical): If a storage key is set to an empty string then to retrieve it you first have to check whether or not that key exist and then check its value. It's not a showstopper by all means, but unnecessary clutter and the removing keys alternative feels like a cleaner approach.

2. (Conceptual): When users explicitly log out they usually expect all information about their visit to be gone. Having empty keys in a session or/and a local storage do not identify users, but the keys presence is information of its own.

I'd love to hear reasons behind your approach. Thank you.