Added taggable behaviour

ha, it can be a string, currently.
Again, I'd prefer a clean signature. I don't think it's the model role of the model to do that stuff (explode comma separated list of tags into an array of Tag objects, f.e). 

It's up to the controller (or wahtaver) to correctly call the model with correct VOs, IMHO.

Thoughts?
It would simplyify the (validateTags and other methods).