Added feature #2023 : Missing function, cv2.boxPoints()

* Added missing feature #2023: cv2.boxPoints(). Ref: http://www.code.opencv.org/issues/2023

Example:

    In [3]: box = ((10,10),(5,5),0)

    In [4]: cv2.boxPoints(box)
    Out[4]:
    array([[  7.5,  12.5],
           [  7.5,   7.5],
           [ 12.5,   7.5],
           [ 12.5,  12.5]], dtype=float32)

* Corrected missing docs for python bindings of cv2.setMouseCallback() and cv2.createTrackbar()