Abstract __query__ to support various backends of table

[kanghyojun]

Now in dodotable __query__ build sqlalchemy.query.Query instance only. however if __query__ is implemented as a some abstract interface, dodotable can choice its way to populate datas from query.

so dodotable can get data from python's dict or list, sqlalchemy query, response JSON get from API call(using requests or any request library) and etc.