CB-6976 Add support for Windows Universal apps (Windows 8.1 and WP 8.1)

https://github.com/apache/cordova-lib/pull/39

* The following two definitions are duplicated for 'windows' platform for backward compatibility with current plugins which could rely on old mechanism based on windows8/commandProxy

    "cordova/windows8/commandProxy"
    "cordova/windows/commandProxy"

* windows8 folder and corresponding gruntfile setting must be removed after 3.6.0 release