Fix Display Width with Unicode library

[5long]

#91 illustrates that long lines containing wide characters causes wrapping problems. Thus this patch.

The newly introduced dependency unicode carries correct display length for unicode charaters.

[gauteh]

Merged.

[gauteh]

I needed a (fix_encoding):

  def display_length
    @display_length ||= Unicode.width(self.fix_encoding, false)
  end

to handle some cases in my Mail test branch, could not reproduce the same in regular RMail. Might be needed for the future though.