Cookie auth storage needs to be cleared on logout

[dillchuk]

Issue:

• user signs in
• session expires (or is deleted) thus uses goalio cookie login
• now logout then register a new user through zfcuser-register UI
• cookie auth logs you back in as the original user (not the new user) since it still has that in storage.

To fix:
Add to Adapter/Cookie.php:

public function logout() {
   //...
   $this->getStorage() and $this->getStorage()->clear();
}

There hasn't been maintenance on this package for quite a while. I'll take admin access, or I may end up forking this into a new packagist package to maintain myself.