Sorted Keypoints

keypoints are sorted according to the distance so user can provide number of best matches to be drawn. There was also an indentation error(weird). solved it.
And used __~img__, got an error -> Image instance has no attribute '__invert__', so added it.

    i=Image("kpimg.png")
    i1=Image("kptemp.png")
    img=i.drawSIFTKeyPointMatch(i1,width=2,num=10)
    img.show()
![sift_match_num](https://f.cloud.github.com/assets/1256649/77745/546575f2-6157-11e2-9a6b-34b639e7a92c.png)

Invert:

    img1 = ~img
    img1.show()