function variance( a, b ) {
	if ( a >= b ) {
		return NaN;
	}
	return (1.0/12.0) * pow( b-a, 2.0 );
}
Returns the variance of a uniform distribution.

@param {number} a - minimum support
@param {number} b - maximum support
@returns {PositiveNumber} variance

@example
var v = variance( 0.0, 1.0 );
// returns ~0.083

@example
var v = variance( 4.0, 12.0 );
// returns ~5.333

@example
var v = variance( -4.0, 4.0 );
// returns ~5.333

@example
var v = variance( 1.0, -0.1 );
// returns NaN

@example
var v = variance( 2.0, NaN );
// returns NaN

@example
var v = variance( NaN, 2.0 );
// returns NaN