Fixes for #30
Fixes for #30

- Sent back the negotitated heartbeat interval in tune_ok

- Made `server_heartbeat` and `client_heartbeat` attributes of the class
  so that they are not dynamically created.

- Renamed `heartbeat_interval` -> `heartbeat` for backwards compatibility.

yetone/py-amqp,dims/py-amqp,smurfix/aio-py-amqp,dims/py-amqp,jonahbull/py-amqp,dallasmarlow/py-amqp,yetone/py-amqp,jonahbull/py-amqp,smurfix/aio-py-amqp,dallasmarlow/py-amqp