add display of genres in playback UI via {genres}

[KUHLwasStolen]

As suggested in #145 this pull request adds the functionality to display genres in the playback UI.

The number of genres is adjustable via the genre_num config option.
{genres} can be used in the playback_format config to place the genres where wished.

The genres are formatted as follows (genre_num = 2)

• 1 genre: "genre1"
• 2 genres: "genre1, genre2"
• >= 3 genres: "genre1, genre2, ..."

Here is an example with the default genre_num = 2

The genres are retrieved for the first/main artist of a track and cached in association to this artist.

closes #145