Implement Disco queries against other entities (#124)

* Improve support for XEP-0030

This commit allows the user to query information about the server
or a node belonging to the server as per XEP-0030.

* Fix broken PubSub functionality

xmpp_disco.go

@@ -0,0 +1,99 @@
+package xmpp
+
+import (
+	"encoding/xml"
+)
+
+const (
+	XMPPNS_DISCO_ITEMS = "http://jabber.org/protocol/disco#items"
+	XMPPNS_DISCO_INFO  = "http://jabber.org/protocol/disco#info"
+)
+
+type clientDiscoFeature struct {
+	XMLName xml.Name `xml:"feature"`
+	Var     string   `xml:"var,attr"`
+}
+
+type clientDiscoIdentity struct {
+	XMLName  xml.Name `xml:"identity"`
+	Category string   `xml:"category,attr"`
+	Type     string   `xml:"type,attr"`
+	Name     string   `xml:"name,attr"`
+}
+
+type clientDiscoQuery struct {
+	XMLName    xml.Name              `xml:"query"`
+	Features   []clientDiscoFeature  `xml:"feature"`
+	Identities []clientDiscoIdentity `xml:"identity"`
+}
+
+type clientDiscoItem struct {
+	XMLName xml.Name `xml:"item"`
+	Jid     string   `xml:"jid,attr"`
+	Node    string   `xml:"node,attr"`
+	Name    string   `xml:"name,attr"`
+}
+
+type clientDiscoItemsQuery struct {
+	XMLName xml.Name          `xml:"query"`
+	Items   []clientDiscoItem `xml:"item"`
+}
+
+type DiscoIdentity struct {
+	Category string
+	Type     string
+	Name     string
+}
+
+type DiscoItem struct {
+	Jid  string
+	Name string
+	Node string
+}
+
+type DiscoResult struct {
+	Features   []string
+	Identities []DiscoIdentity
+}
+
+type DiscoItems struct {
+	Jid   string
+	Items []DiscoItem
+}
+
+func clientFeaturesToReturn(features []clientDiscoFeature) []string {
+	var ret []string
+
+	for _, feature := range features {
+		ret = append(ret, feature.Var)
+	}
+
+	return ret
+}
+
+func clientIdentitiesToReturn(identities []clientDiscoIdentity) []DiscoIdentity {
+	var ret []DiscoIdentity
+
+	for _, id := range identities {
+		ret = append(ret, DiscoIdentity{
+			Category: id.Category,
+			Type:     id.Type,
+			Name:     id.Name,
+		})
+	}
+
+	return ret
+}
+
+func clientDiscoItemsToReturn(items []clientDiscoItem) []DiscoItem {
+	var ret []DiscoItem
+	for _, item := range items {
+		ret = append(ret, DiscoItem{
+			Jid:  item.Jid,
+			Name: item.Name,
+			Node: item.Node,
+		})
+	}
+
+	return ret
+}