Change python game_state structure to a class

[phblj]

Right now game_state is a dict with lots of helper functions. It's a prime candidate to become a class, and those helper functions will become methods.

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Implementation:

create a class GameState(object). It should have the following methods:

• @classmethod load(slug)
• save()
• get_seed_count()
• get_resource_count()
• get_plot(x, y)
• has_planted_crops()
• initialize()
• get_unlocked_plot_count()
• check_password(password)
• jsonify()

The dict storing the data should be private to the class (self._data = {...})

Existing code should be refactored to no longer need direct dict access.

[phblj]

You'll find this issue slightly easier (and will be able to take in a password in the load function) if you merge PR #74 before you start working on it.