function log( x, b ) {
	return ln( x ) / ln( b );
}
Computes the base `b` logarithm of a double-precision floating-point number.

@param {NonNegativeNumber} x - input value
@param {NonNegativeNumber} b - base
@returns {number} logarithm (base `b`)

@example
var v = log( 100.0, 10.0 );
// returns 2.0

@example
var v = log( 16.0, 2.0 );
// returns 4.0

@example
var v = log( 5.0, 1.0 );
// returns Infinity

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

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

@example
var v = log( -4.0, 2.0 );
// returns NaN

@example
var v = log( 4.0, -2.0 );
// returns NaN