Python bindings for KeyPoint methods

1. Added Python bindings for Keypoints method like ``convert()``, ``overlap()`` etc

**Usage:**

    import numpy as np
    import cv2
    img = cv2.imread('messi5.jpg',0)
    sift = cv2.SIFT()
    kp = sift.detect(img)
    
    # Convert KeyPoints to (x,y) coordinates
    pts = cv2.KeyPoint_convert(kp)

    # Convert (x,y) coordinates to keypoints
    kps = cv2.KeyPoint_convert(pts.reshape(-1,1,2))

    # Find overlap between two keypoints
    overlap = cv2.KeyPoint_overlap(kp[0],kp[1])

2. Added docs for the above

3. A small change in docs of ``imshow()`` as per the instruction here: https://github.com/abidrahmank/opencv/commit/fa1b96bd365d89df92cfdceb049bb4accc31a412