Reintroduce ModelList for mass model save/delete actions

I'm using your old ModelList class for mass save actions together with Retrofit. My Retrofit services return `ModelList<? extends Model>` types for which I then can do `service.fetchAll().saveAll();`.

I've also added the static method `from(CursorList<? extends Model> cursorList)` which returns a new ModelList from a CursorList so that you can e.g. delete all models of your query with one call.