change manifest db structure

[pierrel]

Requirements

1. by-key lookup
2. albums with multiple artists
3. no direct db access
4. list artists
5. list albums
6. list songs

[pierrel]

4, 5, and 6 done

for 1, just have the "keys" array changed to a hash that points to artist name, album name, and track

[pierrel]

1 done with a65fa7b