Fetch label.releases_url with new model labelReleases

[4www]

In order to fetch label.releases_url, which is a relationship, but does not exists in the form of a hasMany, we try to create a new model labelReleases.

This new ressources endpoint is releases_url = api.discogs.com/labels/:id/releases

Therefore it allows us to get:

• [x] serialize label.release_url = discogs_api_url_string into label.labelReleases = label.id
• [x] customize labelReleases adapter to have it find the ressource at the right endpoint, under the labels resources
• [ ] serialize labelReleases results, labelReleases.releases = hasMany('releases')
• [ ] display then list of a label's release on its template page

Notes:

1. Maybe using a model is not the right solution for assigning the releases data to a label. I thought it was nice to use ember-data's adapter/serializer for making it work
2. The choice of model name labelReleases might be discussed. The idea here, is to stay close to the resource endpoint name and the hasMany only relation of this model.

[oskarrough]

Just noticed we also have a similar relationship releases_url on artist. Example: https://api.discogs.com/artists/5456