[MRG] setup automated Windows testing on AppVeyor

The setup is simpler than sklearn's as there no need to generate the wheel for the platform and there is no compiled extension in joblib.

This is only useful to run the tests under windows on a regular basis for joblib developers.

Prior to merging, one should remove the `branches:` restriction in `appveyor.yml`. A sample build report is available here:

https://ci.appveyor.com/project/ogrisel/joblib

Currently it points to my own github repo. Once this is merged in master I will update it to watch the github.com/joblib/joblib.git repo instead.