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

https://issues.apache.org/jira/browse/CB-6976

* Renamed windows8 platform to windows. 
* Windows platform can build apps for Windows 8, Windows 8.1 and Windows Phone 8.1 (project is now based on Windows Universal app template).
* There is separate solution just for Windows 8.0 to be able to build platform using Visual Studio 2012 (build script automatically handles this case).
* Update script can update existing windows8 template to a new one (cordova platform update windows8).