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
You could only install php 5.5 and it lacked the sql server driver. Which had to be compiled manually since there is no official driver for 5.5 :(

Also using their installer was noticeable slower than just unzipping it