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 agree with your arguments. 

My arguments:

 - `save(storage, name, null)` is simpler to implement and maintain, as there is less code involved
 - leaving empty keys in the browser storage may make troubleshooting easier

Note that the storage is still local to the page (URL). Even if it was not empty, it could not be accessed by other pages, at least AFAIK.

I don't mind either approach as long as it we make a conscious decision considering all pros and cons.