first draft of data-property getter/setter functions functionality

hi - here is a stab at some documentation for the recently merged property getter/setter functionality ... it's not quite finished yet but I'd like some feedback just to make sure it is adequate as far as it goes.

I have no real idea about how to test documentation changes in the wider context of the docu site - looks like it's a sinatra app but my Ruby-fu is pretty non-existent and there is no README to help idiots like me ... might be worth sticking a little "how-to-run-this" explaination in the sequelize-doc repo?