Will hide nick change/join/part messages, fixes #189

  - Set hide timeout to 30 seconds
  - Changed class="..." to data-event="..."
    Use "data-event" in _chat.scss, inspired by http://glenmaddern.com/articles/introducing-am-css
  - New events:
    - conversation-add
    - conversation-loaded
    - conversation-remove
    - nick-changed
    - nick-joined
    - nick-parted
    - nick-quit