CB-5295 - Fix issue not finding platform script when in subdir

Fix issue not finding platform script (create.js, update.js) when platform is in a subdirectory in its repository
- check platforms which have subdir
- append subdir to libDir for detected platforms

Platforms affected :
- Windows Phone 7
- Windows Phone 8
- Windows 8

I took the initiative to add Windows 8.1 detection as it will be hosted on Windows repository in windows81 subdirectory.