"Normalize" payload before computing signature

- Trello assumes all signatures are binary, which means discarding all
  but the 8 least significant bits for each character. Endash, for
  example, is 8211, but the least significant 8 bits are 00010011, which
  is just "19"
- So, in order to generate the same signature as the trello api does, we
  need to unpack('U*').pack('c*'), which says "unpack as utf-8", and
  then pack as 8-bit

See http://apidock.com/ruby/String/unpack for more information