byte/string management for py3 comptability

* explicitly convert commands to bytes
* use bytes when interacting with the buffer read from socket.

I've only tested this change out manually on python 3.3.2 with a few memcached commands (``get/add/gets/cas``) and it works as expected. The full set of unit tests, however, doesn't run on python 3.3 - and I can look into that with another pull request.