Fix Duel model and get Duel#random_duel working

- Change winning_photo:string field to winning_picture_id
- Fix typo in Duel model (::base vs. ::Base)
- Define Picture#random_ids
- Have Duel#random_duel return a Duel instance
- has_many :duels_pictures -> has_many :duel_pictures
Merge pull request #1 from jfarmer/fix-duel-model

Fix `Duel` model and get `Duel#random_duel` working