Translate models

This PR intends to start a discussion around the details on how the model translations features should work. I've updated the README to describe a little bit what we have so far. Please take a look https://github.com/spree/spree_i18n/tree/translate-models#model-translations

I believe that if this looks good enough it will be pretty easy to add translations for any other Spree model.

Some stuff that is still missing:

* I18n fallbacks

I guess we should use it as default, please see https://github.com/svenfuchs/globalize3#i18n-fallbacks-for-empty-translations. Can you think of reason why users should not have that as default?

* Translate taxonomies / taxons permalinks

Currently only name, description, keywords can be translated. Do you believe we should extend it to slugs as well? I'm not sure this is worth it.

* Persist supported locales via UI

Users can pick which language inputs should be displayed, based on the ``all_locales`` config. However that information is not persisted. Is it worth it to persist that info? I'm not sure about this one either. I don't think language support is something that changes very often on a store. Please advise.

cc @radar @BDQ