Prevent newer instances of `Glicko2` from affecting existing `Player`s

The test I've added should help illustrate this bug.

Before this patch, the `defaultRating` and `volatility_algorithm` of the `Player` class were defined upon the instantiation of a `Glicko2` instance. Thus, new `Glicko2` instances would affect the `defaultRating` and `volatility_algorithm` of already-instantiated `Player`s.

``` js
var glicko = new glicko2.Glicko2({
  rating: 1400
});
var player = glicko.makePlayer();

var newerGlicko = new glicko2.Glicko2({
  rating: 1600
});

player.getRating();
// => 1600
```

As seen in the above code, `player` was created under `glicko`, which sets the `defaultRating` to `1400`. However, the instantiation of `newerGlicko` re-defines `Player.prototype.defaultRating` to `1600`, causing `player`'s rating to change, despite having been created under `glicko`.

This fix moves the definition of `defaultRating` and `volatility_algorithm` to `makePlayer()`. Instead of changing the `prototype` of `Player`, the `prototype` of the specific `Player` created by `makePlayer()` is what's changed.

I've also fixed a couple of typos that I had made in #5.