add 'findContours:contours:mode:method:offset:' method

Added the [findContours](http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=findcontours#findcontours) wrapper method.

I decided to return an array of UIBezierPath, instead of an array of arrays of points.
Let me know what you think. I can adjust things.