all: update copyright message
xmpp: allow handlers to write to the session
This also restructures Session to be an xmlstream.TokenWriter instead of
having a method to return one.
xmpp: back to being unopinionated about handlers
xmpp: remove mux in favor of a middleware like API
xmpp: add experimental ServeMux API
Rework handler API
Also move the handler off the config, which made no sense and remove
concurrency that should not be hidden from the user.
Implement session send/receive API
Fix some copy/pasta copyright dates
Add basic xmpp package docs
Add copyright header to all go files
Update stream creation / add empty Handler
Start working on XMPP/Connection handlers