Make keepalive interval configurable

This fixes #133

config.go

@@ -2,6 +2,7 @@ package xmpp
 
 import (
 	"os"
+	"time"
 )
 
 // Config & TransportConfiguration must not be modified after having been passed to NewClient. Any
@@ -9,12 +10,13 @@ import (
 type Config struct {
 	TransportConfiguration
 
-	Jid            string
-	parsedJid      *Jid // For easier manipulation
-	Credential     Credential
-	StreamLogger   *os.File // Used for debugging
-	Lang           string   // TODO: should default to 'en'
-	ConnectTimeout int      // Client timeout in seconds. Default to 15
+	Jid               string
+	parsedJid         *Jid // For easier manipulation
+	Credential        Credential
+	StreamLogger      *os.File      // Used for debugging
+	Lang              string        // TODO: should default to 'en'
+	KeepaliveInterval time.Duration // Interval between keepalive packets
+	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