Discogs support

I've added support for a user's Discogs collection. This includes:
* cached collection view (title or wall)
* adding scrobbled records to discogs collection if not already present (optional)
* caching of album artwork of user's Discogs collection

I've also updated the search api to comply to the latest Discogs version, barcode searching should work again now. Authentication implemented through OpenAuth, **requires scribe library**

Tested for 6 months without crashes by two people, collections ranging in 100 and 300 records.

Please note that adding your discogs collection is completely optional from a user's perspective, but that *a Discogs API key is required to build the application* from now on (I used a similar process as the Last.fm API key inclusion)

Best regards,
Jono