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.
Not exactly. I wanted to express that the instance is a `SafeYAML::Store` (class) as well as a `YAML::Store` (superclass). I felt that only verifying the latter could be confusing.