Adds release group urls

spec/models/artist_spec.rb

@@ -56,5 +56,6 @@ describe MusicBrainz::Artist do
     release_groups.first.type.should == "Single"
     release_groups.first.title.should == "Club Foot"
     release_groups.first.first_release_date.should == Date.new(2004, 5, 10)
+    release_groups.first.urls[:discogs].should == 'http://www.discogs.com/master/125150'
   end
 end

spec/models/release_group_spec.rb

@@ -21,6 +21,7 @@ describe MusicBrainz::ReleaseGroup do
       release_group.type.should == "Album"
       release_group.title.should == "Empire"
       release_group.first_release_date.should == Date.new(2006, 8, 28)
+      release_group.urls[:wikipedia].should == 'http://en.wikipedia.org/wiki/Empire_(Kasabian_album)'
     end
   end