Convexity Defects support added.

Issue #513
Resending for #514
Added support for far points too.

    img = Image('lenna')
    img = img.threshold(128)
    blobs = img.findBlobs()
    blob = blobs[-1]
    lines, farpoints = blob.getConvexityDefects()
    lines.draw()
    farpoints.draw(color=Color.RED, width=-1)
    img.show()

    points = blob.getConvexityDefects(returnPoints=True)
    startpoints = zip(*points)[0]
    endpoints = zip(*points)[1]
    farpoints = zip(*points)[2]

Results in top left part of the image

![defects_convexity1](https://f.cloud.github.com/assets/1256649/478508/a0b7bec8-b7fc-11e2-88d0-8ba0002f88e4.png)