Only define Promotion::Rules::Product decorator IF Spree::Promotion is defined
Chain scopes instead of merging in Spree::ProductScope#apply_on. Fixes #11.

Fixes #24
Fixed product_group#include?

* If product_groups are added in seeds.rb, it breaks spree_sample:load
  because res.count returns an empty hash. Using Array#length fixes
  this.
Changes the fix to product_group#include?