Converted all but the Scene Kit sample to be iOS playgrounds.

Due to complaints about Apple pulling Mac Swift support from Xcode 6.0 the day the article went live, I've converted all by the Scene Kit playground to be iOS playgrounds instead of Mac ones. The Scene Kit playground doesn't run if I convert it over, so it'll need a little work, but this at least allows readers to run the other three using Xcode 6.0 GM.

Hope I'm sending this in the right direction.