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.

stanza/open.go

@@ -0,0 +1,13 @@
+package stanza
+
+import "encoding/xml"
+
+// Open Packet
+// Reference: WebSocket connections must start with this element
+//            https://tools.ietf.org/html/rfc7395#section-3.4
+type WebsocketOpen struct {
+	XMLName xml.Name `xml:"urn:ietf:params:xml:ns:xmpp-framing open"`
+	From    string   `xml:"from,attr"`
+	Id      string   `xml:"id,attr"`
+	Version string   `xml:"version,attr"`
+}