Fix INVALID_FILE_ATTRIBUTES for x64

 - The GetFileAttributes Win32 API call returns a DWORD, which is a platform independent 32-bit wide
   unsigned integer:
   http://msdn.microsoft.com/en-us/library/windows/desktop/aa364944(v=vs.85).aspx
   It's max value is 4294967295:
   http://msdn.microsoft.com/en-us/library/windows/desktop/aa383751(v=vs.85).aspx
 - FFI::Pointer.new(-1).address is platform dependent and on x64 is 18446744073709551615, which is
   the max value of a 64-bit wide unsigned integer.
 - The INVALID_FILE_ATTRIBUTES constant is a valid return value of GetFileAttributes and is defined in winbase.h as:
   #define INVALID_FILE_ATTRIBUTES ((DWORD)-1)
 - Therefore INVALID_FILE_ATTRIBUTES should be defined as 0xFFFFFFFF
Merge pull request #10 from Iristyle/patch-1

Fix INVALID_FILE_ATTRIBUTES for x64