Add basic SSL error checking.
More refactoring.

- Rename stomp_header_find() to stomp_frame_header_find(); If this is made a
  public function, the user shouldn't need to poke inside the frame struct,
  just pass the frame itself.

- Add function for finding an existing subscription by id.

- stomp_send() should be able to pass a transaction reference.

- Fix stomp_subscribe() to pass the acknowledgement mode and add the
  subscription to the list.

- Make stomp_error() disconnect our end of the socket and attempt a reconnect.
Server frame functions return int instead of void.

Return 0 if the frame passes our sanity checks, -1 otherwise. Don't invoke the
client callback for the frame if it doesn't look sane. Need to sprinkle some
form of logging here...