CP932 support

* PROBREM
When Ruby is >= 1.9, encoding of files are CP932, and files contain
non-Shift_JIS (but CP932) characters like:

  ① その1

then error occurs like:

  charstring.rb:226:in `encode': "\x87@" from Shift_JIS to UTF-8 (Encoding::UndefinedConversionError)

* NOTE
Would it be better to treat Shift_JIS as CP932?