addContainer() renamed addItem(), markDirty()

• Add missing markDirty() method
• ⚠️ BREAKING CHANGE: Renamed the method addContainer() to `addItem(). It is clearer that the added view is in fact a flex item, and not just a flex container.
  • Added by Luc Dion in Pull Request #8
• Add an implementation of the Ray Wenderlich Yoga Tutorial
  • Added by Luc Dion in Pull Request #7
• Add Swift 4.0 support