Add continuous test integration for Windows

[giampaolo]

Similarly to what we currently do on Linux with Travis, that is: running tests on every push.
https://packaging.python.org/en/latest/appveyor.html
Appveyor doesn't seem cool enough to produce exe/wheels and upload then on PYPI though so that part should remain automatized via make.bat.

[giampaolo]

Fixed in 1405e85
It's very slow (about 30 mins to complete a build for all python versions) but it's so cool to have Windows test integration!