v2.0-alpha2

• Adds @NonNull and @Nullable annotations to improve interop with Kotlin.
• In order to return as few nullable objects as possible, the behavior of some methods has changed. In particular, methods like getGroup(int) and getItem(int) which used to return null for missing Group now throw IndexOutOfBoundsException.
• Group.setGroupDataObserver is now register/unregisterGroupDataObserver.
• The example project has been rewritten in Kotlin with Android extensions.
• The standard library (no databinding) Item.createViewHolder now has a default implementation. The main reason is that if you use Kotlin Android extensions, you never need to create your own ViewHolders—just like the original implementation.