Separate backup

Recommended to review by commit as I try to structure it as refactoring goes. My intention is also to use it as tutoring material for some refactoring presentation.
Just for the record. An approach I have seen in some libraries out there:

```ruby
module BootRecordBackup
  class Backup
      def initialize(device); @device = device; end

      def create; true; end

      private

      def copy_br(target_map); true; end
   end

   def self.create_for(device)
      Backup.new(device).create
   end
end
```