Implement SQL inner join

## SQL joins

Given the following mapping:

```ruby
Lotus::Model::Mapper.new do
  collection :users do
    entity User
    repository UserRepository

    attribute :id,   Integer
    attribute :name, String
  end

  collection :orders do
    entity Order
    repository OrderRepository

    attribute :id,      Integer
    attribute :user_id, Integer
    attribute :total,   Integer
  end
end
```

### Inner join

```ruby
class OrderRepository
  include Lotus::Repository

  def self.foo
    query do
      join(:users)
    end
  end
end
```

```sql
SELECT `orders`.* FROM `orders` INNER JOIN `users` ON (`users`.`id` = `orders`.`user_id`)
```

Closes #34