3.0 windows ci

This adds the appveyor.yml file which is required for the windows ci server. ~~It does not work like travis, though. We will not see feedback from it on pull requests, for example~~. Results can be seen here:

https://ci.appveyor.com/project/cakephp/cakephp-893
A zip that I uploaded containing a php 5.5 install with the sql server drivers. Unfortunately using the php installer that they have was not possible.