Convexity Defects support added.

Issue #513

    img = Image('lenna')
    img = img.threshold(128)
    blobs = img.findBlobs()
    blob = blobs[-1]
    blob.getConvexityDefects().draw(width=5)
    img.show()
 
I have drawn the defects in green (in the right) using this function.

![defects_convexity](https://f.cloud.github.com/assets/1256649/474211/253167ee-b762-11e2-888a-45285120dc38.png)