[Shuffle Friends]

- Remove game functionality and spec
- Adding a friend twice to a game spec
- FriendAssignment model and spec
- Shuffler Service to shuffle the game's friends and create the corresponding friend assigments
- Shuffler Service Spec
- notify_assigment method added to friend_notifier mailer: to send an email informing an user which friend should give a present to
- Shuffling friends feature spec