Add Taggable module (wincent.dev, 003f9f3)

Add a very simple Taggable module designed for use with model classes. The public interface consists of "tag" and "untag" methods (for adding and removing tags) and a "tag_names" method (for getting an array of tag names that apply to a model instance).

Signed-off-by: Greg Hurrell <greg@hurrell.net>