rabbitmqstomp: Fix library for production

- Fix send()
- Add subscribe(), unsubscribe(), receive()
- Complete doc in README
- Remove example, add example in README itself

Test: Requests of 200 concurrent users against 1000 requests/thread, 200K payload size
Result: Complete reliable publishing with confirms with a rate of 400+ msg/second

Signed-off-by: Rohit Yadav <rohit.yadav@wingify.com>