Add XMPP Addon

Follow up of travis-ci/travis-core#357

The XMPP Addon can join to multiple rooms or send to multiple user. It
needs credentials (jid and password) to connect to a XMPP server.

The configuration format looks like : 

```yaml
notifications:
  jabber:
    jid: 'awesome_travis_user@example.com'
    password: 'password'
    rooms:
      - jid: mainhall@example.com/anAwesomePseudo
      - jid: secretroom@example.com/TravisBot
        password: 'password'
    users:
      - one@example.com
      - two@example.com
    template:
      - "%{repository} (%{commit}) : %{message} %{foo} "
      - "Build details: %{build_url}"
```