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.
Is this necessary, given that you're also including store.yaml itself in this commit? I would either:

1. Get rid of the logic to explicitly write the file, or
2. Get rid of the file itself and add it to .gitignore

My preference is for 1, since I don't really see the benefit of 2.