Added method banned_until ban strategy

#### Features
* Added method `banned_until` to know the date the user is banned.
```bash
127.0.0.1:6379> TTL 'development/c5308:rack::attack:23508776:allow2ban:count:127.0.0.1'
(integer) -1
```

This sucks. The `EXPIRE` command does not event work!