exposed more gm functions + fixed clone memory leak 

A few more GM functions have been exposed:
* MagickBorderImage
* MagickColorFloodfillImage
* MagickSetImageBackgroundColor
* MagickCompositeImage
* MagickFlattenImages
* MagickBlurImage
* MagickAddNoiseImage

There was also a memory leak in Image:clone() which has been fixed.