Remember user address in checkout

First draft for my suggestion on remembering user addresses on checkout. I believe the general idea would be to save the user address the first time it's entered and on the next checkout remember that address.

With this patch merged and the `Spree::Core::User::Address` included on your `Spree.user_class` that is now possible. I'm not sure if making the `persist_order_address` is better inside the controller or tied to some other model  method (e.g. as we do with the `use_billing` flag for shipping addresses). For now I went to the controller approach.

Thoughts? Depending on how this goes I might go on with more specs and maybe letting users choose whether or not they want the address associated with their account (something like a checkbox saying "associate address with my account")

any feedback much appreciated