Automatic binary string decoding

- A new parameter **decode_str** for RPCStream connect() and spawn()
  enables automatic decoding of binary strings into Unicode objects
- Strings are decode using the &encoding option
- The default behaviour is decode_str=False, i.e. binary strings are
  returned as type bytes (str for Python2)
- For Python3 unit tests now default to decoding to Unicode, allowing
  the unit tests code to remain unchanged