WinRT support

[Aaron Simmons]

dirent.h doesn't support WinRT for a few reasons:

• #including windef.h directly causes fatal error C1189: #error : "No Target Architecture"
• FindFirstFileW isn't available
• GetFullPathNameW isn't available

I think WinRT support can be added while maintaining compatibility with Win32:

• #include windows.h instead of windef.h
• Replace FindFirstFileW with FindFirstFileExW
• Replace GetFullPathNameW with _wfullpath