function reverseArguments(fn) {
    return function(x, y) { return fn(y, x); };
  }
Takes a function and returns a function with the same logic but the
arguments reversed. Only applies to functions w/ arity=2 as this is private
and I can do what I want.

@private
@param {Function} fn The function to "reverse"
@returns {Function} The "reversed" function

@examples
reverseArguments(function(x, y) { return x + y; })('a', 'b'); // => 'ba'