Enforce Unicode encoding in MSVC

This can help to solve compilation issues on systems with Japanese
locale with encodings like Shift_JIS and UTF-8-BOM.

Also be more consistent using String::utf8() to represent cyrilic
unicode characters in test_string.cpp

Clarified some comments in test_string.cpp for some Unicode characters.

SConstruct

@@ -327,6 +327,8 @@ if selected_platform in platform_list:
         env.Append(CCFLAGS=['/EHsc'])
         if (env["werror"]):
             env.Append(CCFLAGS=['/WX'])
+        # Force to use Unicode encoding
+        env.Append(MSVC_FLAGS=['/utf8'])
     else: # Rest of the world
         shadow_local_warning = []
         all_plus_warnings = ['-Wwrite-strings']