CPAnimation setAnimationCurve: Can Assign Invalid _animationCurve

Here's a modified version of setAnimationCurve: that resolves #175 CPAnimation setAnimationCurve: Rejects CPAnimationLinear and #176 CPAnimation setAnimationCurve: Can Assign Invalid _animationCurve:

- (void)setAnimationCurve:(CPAnimationCurve)anAnimationCurve
{
    var timingFunctionName;
    
    switch (anAnimationCurve)
    {
        case CPAnimationEaseInOut:  timingFunctionName = kCAMediaTimingFunctionEaseInEaseOut;
                                    break;
                                    
        case CPAnimationEaseIn:     timingFunctionName = kCAMediaTimingFunctionEaseIn;
                                    break;
                                    
        case CPAnimationEaseOut:    timingFunctionName = kCAMediaTimingFunctionEaseOut;
                                    break;
                                    
        case CPAnimationLinear:     timingFunctionName = kCAMediaTimingFunctionLinear;
                                    break;
                                    
        default:                    [CPException raise:CPInvalidArgumentException
                                                reason:"Invalid value provided for animation curve"];
                                    break;
    }
    
    _animationCurve = anAnimationCurve;
    _timingFunction = [CAMediaTimingFunction functionWithName:timingFunctionName];
}