Add fpm_attributes DSL method

This is the follow-up PR for #75.

Provides new ``fpm_attributes`` DSL. See the example how it works.
 
```
class Recipe < FPM::Cookery::Recipe
  platform :redhat do
    fpm_attributes :rpm_user => 'httpd'
  end

  platform :debian do
    fpm_attributes :deb_user => 'apache'
  end
end
```