Added Chocolatey packaging utilities

Everything needed to update the Chocolatey package.
(More information : https://github.com/chocolatey/chocolatey/wiki/CreatePackagesQuickStart)

Currently the package is unlisted because it's awaiting moderation, but it's avaible here : https://chocolatey.org/packages/syncthing-gtk

Since I am currently the only maintainer of the package on the website, just create an account on Chocolatey and add it to the list of `owners` in the `.nuspec`file. I'll push it as soon as I can, so you will have an access too.
I used Python3 since most developpers have it already installed along Python 2, and I'm more familiar with it, but if you want I'll look into making it working with Python 2.