Fix s-reverse for Unicode combining characters.

When reversing a Unicode (multibyte) string, care must be taken to
reverse characters with their combining marks as a single unit,
maintaining their order.

* https://github.com/mathiasbynens/esrever
* http://www.cl.cam.ac.uk/~mgk25/unicode.html
Merge pull request #58 from skeeto/fix-reverse

Fix s-reverse for Unicode combining characters.