Add game management

[armandomiani]

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[amsoares]

To make it consistent with the whole idea of keeping the code controlling the whole game, we should also implement an array of "already used words" per category.

something like:

"category": "color",
"said": ["blue", "blank", "beige"]

and some methods that would verify:

boolean isUsedWord = game.isUsedWord(word);
boolean isValidInitial = game.isValidInitial(word);