Make use of libevent evdns resolver.

- Finally remove the hardcoded struct sockaddr_in and actually use what is
  passed to stomp_connection_new().

- Adjust stomp_init() to allow passing of an existing evdns_base *, otherwise
  create one with sane defaults from the passed event_base *.

- Replace bufferevent_socket_connect() with
  bufferevent_socket_connect_hostname() and add some error checking in the
  event callback for DNS errors.

- Fix up everywhere where the port was specified as a signed short and instead
  should be unsigned.