function isNegativeInteger( x ) {
	return Boolean( addon( x ) );
}
Tests if a finite double-precision floating-point number is a negative integer.

@private
@param {number} x - value to test
@returns {boolean} boolean indicating whether the value is a negative integer

@example
var bool = isNegativeInteger( -3.0 );
// returns true

@example
var bool = isNegativeInteger( 4.0 );
// returns false