REFACTOR use WhereChain

In order to make it clear what is actually merged with `#any_of` is
conditions only, `#any_of` and `#none_of` now sits behind WhereChain :

```ruby
inactive_users = User.where.any_of( { banned: true }, "confirmed_at = NULL" )
newsletter_users = User.where.none_of( inactive_users, { subscribed: false } )
```

Previous behavior has been kept for rails-3, as WhereChain does not
exist there. A deprecation warning has been added for rails-4, so that
users have time to migrate.

Close #7
Merge pull request #10 from oelmekki/use_where_chain

REFACTOR use WhereChain