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.
Just one question... Wouldn't be simpler solution just to do `from __future__ import unicode_literals` (and probably print_function) as rest of ST-GTK does? ST-GTK requires py2 and installing _two_ pythons to create package sounds little overboard.