src: implement socket.connect

- Implementing connect in dgram sockets allow us to have some kind of flow control
  as we'll receive EAGAIN in case the server buffer is full and a WRITABLE event
  once it's not.
- Three new events are added:
  · `connect`: if connect succeeds
  · `congestion`: If the server buffer is full
  · `writable`: When the server accepts data again.
- Add test.
- Update README.
- It tries to address bug #4
Merge branch 'santigimeno-drain'