switch to python3 :

- remove encoding parameter (ext-string is bytes())
- rename bytes to buf
- handle bytes<->string decoding/encoding when needed
- rename decode functions to take decimal representation of erlang types
  instead of ascii
- replace __cmp__ implementation in types with __eq__ and __ne__
update tests for python3 version
Add erlang port interface

2 functions port_req generator and port_res
explanations added in README.md