Transports need to handle open/close stanzas

XMPP and WebSocket transports require different open and close stanzas. To
handle this the responsibility handling those and creating the XML decoder is
moved to the Transport.

config.go

@@ -1,7 +1,6 @@
 package xmpp
 
 import (
-	"io"
 	"os"
 )
 
@@ -18,6 +17,5 @@ type Config struct {
 	ConnectTimeout int      // Client timeout in seconds. Default to 15
 	// Insecure can be set to true to allow to open a session without TLS. If TLS
 	// is supported on the server, we will still try to use it.
-	Insecure      bool
-	CharsetReader func(charset string, input io.Reader) (io.Reader, error) // passed to xml decoder
+	Insecure bool
 }