Support for Traco as i18n db backend

Hi !

In one of projects in the company I'm working for, we are using ActiveAdmin with [Traco](https://github.com/barsoom/traco) as i18n backend instead of globalize. The main difference is that i18n fields are in the same table as the model.
So, I decided to add support for Traco in activeadmin-translate.

Hope this will be useful for other people.