Add constants (enumlike) for stanza types and simplify packet creation (#62)

* 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

auth.go

@@ -33,7 +33,7 @@ func authPlain(socket io.ReadWriter, decoder *xml.Decoder, user string, password
 	fmt.Fprintf(socket, "<auth xmlns='%s' mechanism='PLAIN'>%s</auth>", nsSASL, enc)
 
 	// Next message should be either success or failure.
-	val, err := next(decoder)
+	val, err := nextPacket(decoder)
 	if err != nil {
 		return err
 	}