ADD #none_of

`#none_of` is the negative version of `#any_of`. This will return all
active users :

```ruby
banned_users = User.where(banned: true)
unconfirmed_users = User.where("confirmed_at IS NULL")
active_users = User.none_of(banned_users, unconfirmed_users)
```
Merge pull request #4 from oelmekki/none_of

ADD #none_of