Now with added_at and added_by fields

Hi! I'm a great fan of Spotify and recently found out there's this really nice API wrapper in Ruby. 

I started learning it for about a week and while looking at some stuff to do for this project I noticed issue https://github.com/guilhermesad/rspotify/issues/22 was never done so I thought it would be a good one to tackle, specially because you provided some initial ideas in there.

I had to update some yml files because the current ones did not have the new fields, which resulted in some tests failures so I ended up having to fix them.

I believe this might not be the desired version but it's at least functional and might help me get there if you give me some more guidance. I'm also interested on what you think about the code and where it could be improved (i.e. playlist.rb lines 89 - 95 could be merged with line 85 if there's any performance concerns, etc..). 

thanks!