Add fpm_attributes DSL to recipe.

This recipe directive allows user to modify fpm packager driver
specific attributes inside from their recipe accordingly.

class Recipe < FPM::Cookery::Recipe
  platform :redhat do
    fpm_attributes :rpm_user => 'httpd'
  end

  platform :debian do
    fpm_attributes :deb_user => 'apache'
  end
end
inherites fpm_attributes data set from parent classes.
Allows argument assignemnt.
workaround for 1.8.7.

Nil error from @fpm_attributes came out because Ruby 1.8.7 calls
Class.inherited() after the block is evaluated.

Class.new(FPM::Cookery::Recipe) do
  # here comes before FPM::Cookery::Recipe.inherited() runs.
  fpm_attributes # => returns nil
end
Merge pull request #80 from unakatsuo/add-fpm-attributes

Add fpm_attributes DSL method