Resolve "Stickers"

Closes #28 (closed)

TODO:

• modify SQL schema
• functions to manipulate the db
• implement functions in handle_simple_command

For karas, the {TYPE} must be song, for playlists, the {TYPE} must be plt.

A list of the commands to implement:

• sticker get {TYPE} {URI} {NAME}
• sticker set {TYPE} {URI} {NAME} {VALUE}
• sticker delete {TYPE} {URI} [NAME]
• sticker list {TYPE} {URI}
• sticker find {TYPE} {URI} {NAME}
• sticker find {TYPE} {URI} {NAME} = {VALUE}