Making the gpr_tmpfile's win32 version a bit more Windows-y.

Also adding windows helpers to convert to and from TCHAR strings.
diff --git a/vsprojects/vs2013/gpr.vcxproj b/vsprojects/vs2013/gpr.vcxproj
index d1f7085..8276082 100644
--- a/vsprojects/vs2013/gpr.vcxproj
+++ b/vsprojects/vs2013/gpr.vcxproj
@@ -100,6 +100,7 @@
     <ClInclude Include="..\..\src\core\support\file.h" />
     <ClInclude Include="..\..\src\core\support\murmur_hash.h" />
     <ClInclude Include="..\..\src\core\support\string.h" />
+    <ClInclude Include="..\..\src\core\support\string_win32.h" />
     <ClInclude Include="..\..\src\core\support\thd_internal.h" />
   </ItemGroup>
   <ItemGroup>