function beta( a, b ) {
	return addon( a, b );
}
Evaluates the beta function.

@private
@param {NonNegativeNumber} a - input value
@param {NonNegativeNumber} b - input value
@returns {number} evaluated beta function

@example
var v = beta( 0.0, 0.5 );
// returns Infinity

@example
var v = beta( 1.0, 1.0 );
// returns 1.0

@example
var v = beta( -1.0, 2.0 );
// returns NaN

@example
var v = beta( 5.0, 0.2 );
// returns ~3.382

@example
var v = beta( 4.0, 1.0 );
// returns 0.25

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