Added RubyMotion example

Proof-of-concept RubyMotion app using Realm. This is certainly in its infancy and I haven't tried pushing this very far, but it should be a good starting point.

Big :+1: to @alloy for getting this on the right track.

/cc @kneth @timanglade @oleks
Will do. I was doing this for consistency with all other models in this repo, but as discussed elsewhere, we should update all other models to reflect this as well at some point.