give ->order_by() column default value

I use a lot of jQuery's sortable lists all the time and I use the SQL table column 'order_by' as a standard so to stop writing ->order_by( 'order_by' ) I added that as a default value.

This means you would need to name your SQL column 'order_by' to make it work for you. 
The same way you do for 'created_at' or 'updated_at'.

Tested in my own application with plenty of order_by() statements and works fine.