Update to latest version of nanomsg

This commit:

  * updates the interface to the latest version of `nanomsg`
  * makes tests work with `unittest`
  * adds a Makefile

I have used `nn_symbol` to get the symbol from the C lib automatically.

I have bumped the version of cython to 0.18 to leverage improvements in the parser.

Warning: I had never written cython code before this morning so there might be mistakes, I didn't really check the content of socket.pyx either.