Add SafeYAML::Store

A [YAML::Store](http://ruby-doc.org/stdlib/libdoc/yaml/rdoc/YAML/Store.html) variant that uses `SafeYAML.load` instead of `YAML.load`.

Would you be willing to integrate such a thing? If so, I would also try to add some specs for this.
I think you can scrap this particular test. You're basically just testing RSpec here, right?