Serialize with JSON

This adds dependency to json-glib. This needs !73 and !72 merged ;)

An example extracted from tests:

{
  "album" : "A Night At The Opera",
  "artist" : "Queen",
  "performer" : [ "Brian May", "Roger Taylor", "Freddie Mercury", "John Deacon", "Paul Rodgers", "Adam Lambert" ],
  "creation-date" : "2015-09-16T18:06:14+00:00",
  "grl-related-keys": [
    {
      "artist": "first artist",
      "mb-artist-id": "bf24ca37-25f4-4e34-9aec-460b94364cfc"
    },
    {
      "artist": "second artist",
      "mb-artist-id": "6b28ecf0-94e6-48bb-aa2a-5ede325b675b"
    },
    {
      "artist": "third artist",
      "mb-artist-id": "84a87594-8c48-4865-9ac2-5d8aa29c800a"
    }
  ],
}