Add support for reconnecting to a broker.

- Move connection-establishing code to a timer-triggered callback which
  allows use by both the initial connect attempt and any future
  reconnect attempts.

- Add additional timeval parameter to stomp_connect() which specifies
  how long to wait between reconnects.

- On an error an immediate reconnect is attempted, then the
  user-specified delay is used between future attempts. This delay is
  reset once a connection is established.