Deprecate Spree::Address#empty?

[jhawthorn]

Empty is a confusing method to have on an object because it affects the behaviour of blank? and present?

> Spree::Address.new.present?
false

We should remove empty to avoid this confusion

It's currently defined as

def empty?
  attributes.except('id', 'created_at', 'updated_at', 'country_id').all? { |_, v| v.nil? }
end

Which is basically the same as new_record? && country_id.nil?

This fixes #416

[cbrunsdon]

Good by me, thanks.