Record Sets should delegate to class singleton

```ruby
class Post < Cequel::Model::Base
  belongs_to :blog
  key :id, :timeuuid
  column :title, :text

  def self.today
    between(Time.now.beginning_of_day..Time.now.end_of_day)
  end
end
```

Would allow you to do something like:

```ruby
Post['big-data'].select(:id, :title).today
```

Which would return a record set encapsulating the query:

```sql
SELECT id, title FROM posts
WHERE blog_permalink = 'big-data'
  AND id > '2013-09-12 00:00:00'
  AND id < '2013-09-12 00:00:00'
```