* Add constants (enumlike) for stanza types
* NewIQ, NewMessage and NewPresence are now initialized with the Attrs struct
* Update examples
* Do not export backoff code. For now, we do not need to expose backoff in the documentation
* Make presence priority an int8
- Support for exponential backoff on reconnect to be gentle on the server.
- Clean up client by moving metrics and retry strategy to the connection manager.
- Update echo_client to use client manager
- Fix echo client XMPP message matching
Fixes#21
Improvements for #8
