Update to Rails4 and Globalize4

I've taken and cleaned commits from other contributors and merged with a complete clean and update of the project.

It still lacks tests, so we need to human test it :)

I've changed the homepage of the project to this: http://github.com/cantierecreativo/activeadmin-globalize
I think it would be nice to fork your project to that after the pull request. Than in your Readme we make clear that the project will be updated under cantierecreativo.

What do you think?