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.
Right, the file is not needed. I started out that way because there were already YAML files to test. But `YAML::Store` would overwrite the file after finishing the transaction. Then again, changing to a readonly transaction would require digging deeper into `YAML::Store` internals. That's why I added the `before` hook. I just forgot to remove the file from Git; in which case it should also be cleaned up in an `after` hook.