Update README w.r.t. s/stomp_header_find/stomp_frame_header_find/g.
STOMP 1.0 & 1.1 compatibility.

- Update stomp_ack() and stomp_nack() to require the subscription struct to
  satisfy the 1.1 specification. The ack argument becomes dual-purpose; in the
  1.2 case it should be the value of the "ack" header, in the 1.0 & 1.1 case it
  should be the value of the "message-id" header.

- Update stomp_nack() to be a no-op for 1.0.