Artist analytics

Push analytics events to Google Tag Manager when an artist
is favorited or played (via YouTube or Spotify).

When an artist is favorited, an event with the following data layer variables is sent:

    event:          "setFavorite"
    artistName:     "<name of the artist>"
    isFavorite:     bool, whether the artist was favorited or de-favorited

When an artist is played, an event with the following data layer variables is sent:

    event:          "playArtist"
    artistName:     "<name of the artist>"
    medium:         "<medium, either 'youtube' or 'spotify'>"

# BIG HUGE WARNING

This is dependant on pull request https://github.com/futurice/festapp-android/pull/64 so please don't merge this before PR 64. Also note the caveats of PR 64.