Move "public" methods to Body.prototype.

Hopefully, this will make extending Body more straightforward.

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Use initBody() to initialize Body from constructor.

This way, we can sub-class Body and call initBody() from our sub-class
constructor.  This may be a very stupid idea but I'm still learning how
to make best use of JavaScript's inheritence system.

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Use config object for Body constructor arguments.

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Add Math.randomFloat() to utils.js.

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Add Human class (Body sub-class).

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