Merge branch 'hotfix-0.11'
Merge branch 'master' of https://github.com/raphiniert/AnyEvent-STOMP-Client into develop
Implemented callbacks on_send, on_ack and on_nack for the corresponding STOMP commands SEND, ACK, NACK.
Documented new callbacks in the documentation.
Merge branch 'feature_#11-more_callbacks' into develop