[WIP] Add jabber notifications travis-ci/travis-ci#958

Hi Everybody !

This is my first try at implementing Jabber/XMPP notifications. Some information about this implementation : 

- I've tested the whole Client/Task duo and it works for both Room and User notification :v: 
- It uses the gem [XMPP4R](https://github.com/xmpp4r/xmpp4r), this is criticizable, so let me know if you need an implementation from scratch (which not easy).
- I didn't test the "whole process", from a successful build to a Jabber notification. I will need some help here.
- I didn't handle the exceptions, I looked at the other Services (like Flowdock) and it doesn't seem to do it too, is it a design choice ? I could be plain wrong, and I will be glad to implement the needed rescues.
- I don't find my specs to be particularly insightful. I will try to add more useful ones.
- The user need to have a Jabber account. If Travis chooses to provide its owns it could be optional.

The configuration format support two kind of notifications : rooms (with or without passwords) and users.

It looks like this : 
```yaml
notifications:
  jabber:
    jid: 'awesome_travis_user@example.com'
    password: 'password'
    rooms:
      - jid: mainhall@example.com/anAwesomePseudo
      - secretroom@example.com/TravisBot
        password: 'password'
    users:
      - one@example.com
      - two@example.com
    template:
      - "%{repository} (%{commit}) : %{message} %{foo} "
      - "Build details: %{build_url}"
```