Fixes to data model, replaced ManyToMany relationship for a ListField
containing the indexes to the cards (as ManyToMany would require a JOIN
operation which is forfeited in NoSQL database like AppEngine Datastore

Added minimal test case that validate game data model construction.
Some other ManyToMany replacements
Adding basic unit tests validating model functions
Fixed an error on test test_game_next_storyteller_when_no_rounds.
Added functionality to handle first part of the game: initiate a game
round, allow each player to play a card.