[WIP] configure Windows CI with AppVeyor

At the moment I have registered this branch on my ogrisel account:

https://ci.appveyor.com/project/ogrisel/numpy

As you can see there are 9 errors in the tests caused by deprecation warnings. I have not investigated why it's happening. I don't have a windows host at hand at the moment.

If we want to merge this, we should create a shared appveyor account on the ci.appveyor.com service and share the password among core numpy developers.

If we want to have appveyor to monitor all incoming pull request in complement to travis, we would need to setup multi-status github reporting:

http://github-multi-status.herokuapp.com/

Otherwise it's possible to configure appveyor to run the test on commits merged to master only instead.

@cournape is experimenting with building numpy against libopenblas.dll to be able to skip the liblapack_lite build step that is very slow on appveyor.
Also the original configuration for appveyor is maintained in this repo:

https://github.com/ogrisel/python-appveyor-demo

and a variant is documented in the PyPA official documentation:

https://packaging.python.org/en/latest/appveyor.html