2018-08-23 Build protobuf and onnx for Windows

A good way to build it is to look into the file appveyor.yml. You should end up with something like this: build_protobuf.bat. onnx was a little bit more complex to build as the current build system is based on Miniconda and there is still no version for Python 3.7. I had to start with the standard distribution which I did. The release is available as artifacts on appveyor/onnx. Instructions can be found in appveyor.yml.