I18n

This PR is just for ensuring that the translations of Spree are namespaced. Nothing else past that.

Other things that would be good to do:

 - [ ] Move Spree translations over to Localeapp (need @schof to add spree, spree_i18n and all other Spree extensions to this).
 - [ ] Decide if namespacing would be enough, or if we want to go even further and group the translations into logical blocks. Right now it's just one big junk drawer of translations, not that there's a problem with that current approach (yet!)
 - [ ] Update extensions with the new `Spree.t` helper.