Add new XMPP Notification plugin

Add new XMPP notification plugin of successful / failure build. 

example configuration :

```
complete:
    xmpp:
        recipients:
            - "recipient1@jabber.org"
            - "recipient2@jabber.org"
        tls: true
        username: "yourxmppaccount@server.com"
        password: "password"
```

Use «server» param name to define another server :

```
complete:
    xmpp:
        recipients:
            - "recipient1@jabber.org"
            - "recipient2@jabber.org"
        tls: true
        username: "yourxmppaccount@server.com"
        password: "password"
        server: "gtalk.google.com"
```

Use «alias» to define one pseudo 

Use «date_format» to define date format (strftime)