Quite messy pull request with some features included

Hi there,

I found your gem and had to edit many things to make it work for my case, I'm going to list what I changed and you'll tell me if you want to merge this PR as is or if you prefer I split my commit in multiple ones. 
I'll understand the latter while I'll prefer the first *heh*.

1. [Rich editor](https://github.com/bastiaanterhorst/rich) support for text areas
2. Less restriction on `globalize` and `rails_admin` gem versions. RailsAdmin is moving quite fast and keeping restrictive versions disallows to use your gem without having to update the continuously gemspec
3. Use `field.label` instead of `field.name` as the field title for each translation field, so we let RailsAdmin handle translations for it.
4. Included french translations : since I saw you already included english and italian ones, I added the french ones since I needed it and they can be useful as the others.