WIP: Merge in upstream

This MR incorporates the latest upstream changes, which should enable use of YOLOv3, among other things.

There are a number of backwards-incompatible changes, including but not limited to:

• Various functions that now take or return network * instead of network
• An extra argument for show_image that is used to call cv::waitKey
• The removal of get_region_boxes, replaced by get_region_detections, which uses the new detection type
• The use of void * instead of various OpenCV types

My changes can be classified into roughly two categories:

• Those that modify the CMake code to correspond to upstream changes
• Those that fix upstream issues

I've limited the number of upstream-related changes that I made beyond those needed to get the important parts working, which means that certain files in the examples directory that require OpenCV don't build (due to being left behind when upstream made the OpenCV changes mentioned above); I've worked around this by adding a BUILD_EXAMPLES option to the CMake code, defaulting to false, that causes the examples to be built.

I've marked this MR as WIP mainly because it needs to be validated on Windows.