Renamed subroutine receive_frame to read_frame.
Added checks for acknowledgement mode in subscribe subroutine.
Added warning to include content-type headers when sending a message.
Modified unsubscribe function: Unsubscribe with destination string instead of id.
Added function to check validity of a destination string. Adapted all related functions accordingly.
Added more (polite) error handling.
Fixed bug, which disallowed to unsubscribe and subscribe again.
Added dummy functions to register on_subscribe and on_unsubscribe callbacks.
Bugfix of the prior bugfix.
Implemented events on_subscribed() and on_unsubscribed() (closes #6).
Events are only fired if there is a callback registered on these events.
Receipt ID will be generated if not explicitly given.
Furthermore added support for custom additional headers.