Add hook for admin logout

Osclass have a hook when an administrator logs on to the web, I added hook when logout.

``` php
osc_run_hook('logout_admin');
```

The hook name is based on the login hook.

``` php
osc_run_hook('login_admin', $admin);
```

No parameter is passed to the hook like in the frontend hook

``` php
osc_run_hook("logout");
```