PubSub protocol support (#142)

* PubSub protocol support
Added support for :
- XEP-0050   (Command))
- XEP-0060   (PubSub)
- XEP-0004   (Forms)

Fixed the NewClient function by adding parsing of the domain from the JID if no domain is provided in transport config.
Updated xmpp_jukebox example

* Delete useless pubsub errors

* README.md update
Fixed import in echo example

* Typo

* Fixed raw send on client example

* Fixed jukebox example and added a README.md

stanza/jid.go

@@ -0,0 +1,91 @@
+package stanza
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+type Jid struct {
+	Node     string
+	Domain   string
+	Resource string
+}
+
+func NewJid(sjid string) (*Jid, error) {
+	jid := new(Jid)
+
+	if sjid == "" {
+		return jid, fmt.Errorf("jid cannot be empty")
+	}
+
+	s1 := strings.SplitN(sjid, "@", 2)
+	if len(s1) == 1 { // This is a server or component Jid
+		jid.Domain = s1[0]
+	} else { // Jid has a local username part
+		if s1[0] == "" {
+			return jid, fmt.Errorf("invalid jid '%s", sjid)
+		}
+		jid.Node = s1[0]
+		if s1[1] == "" {
+			return jid, fmt.Errorf("domain cannot be empty")
+		}
+		jid.Domain = s1[1]
+	}
+
+	// Extract resource from domain field
+	s2 := strings.SplitN(jid.Domain, "/", 2)
+	if len(s2) == 2 { // If len = 1, domain is already correct, and resource is already empty
+		jid.Domain = s2[0]
+		jid.Resource = s2[1]
+	}
+
+	if !isUsernameValid(jid.Node) {
+		return jid, fmt.Errorf("invalid Node in Jid '%s'", sjid)
+	}
+	if !isDomainValid(jid.Domain) {
+		return jid, fmt.Errorf("invalid domain in Jid '%s'", sjid)
+	}
+
+	return jid, nil
+}
+
+func (j *Jid) Full() string {
+	return j.Node + "@" + j.Domain + "/" + j.Resource
+}
+
+func (j *Jid) Bare() string {
+	return j.Node + "@" + j.Domain
+}
+
+// ============================================================================
+// Helpers, for parsing / validation
+
+func isUsernameValid(username string) bool {
+	invalidRunes := []rune{'@', '/', '\'', '"', ':', '<', '>'}
+	return strings.IndexFunc(username, isInvalid(invalidRunes)) < 0
+}
+
+func isDomainValid(domain string) bool {
+	if len(domain) == 0 {
+		return false
+	}
+
+	invalidRunes := []rune{'@', '/'}
+	return strings.IndexFunc(domain, isInvalid(invalidRunes)) < 0
+}
+
+func isInvalid(invalidRunes []rune) func(c rune) bool {
+	isInvalid := func(c rune) bool {
+		if unicode.IsSpace(c) {
+			return true
+		}
+		for _, r := range invalidRunes {
+			if c == r {
+				return true
+			}
+		}
+		return false
+	}
+	return isInvalid
+}