Contours expansion: minAreaRect and convexHull bindings, expose underlying point data

Hey there! I needed access to some additional functions in OpenCV, so I went ahead and added a few more bindings to the Contours class.

`Contours.minAreaRect` maps to `cv::minAreaRect`, and returns an object describing the size, angle, and points of the RotatedRect.

`Contours.convexHull` maps to `cv::convexHull`, and takes the `clockwise` boolean as a parameter.

`Contours.point` allows access to the coordinates of the individual points that make up the contour.