Return album, artist, and song IDs in JSON

[MarcelloTheArcane]

Following on from #121, it would be really useful if search endpoints could return the ID of the album, artist and song (the song can be parsed from the location property, but it would be nice to have this on its own too).

Example response for /get_by_search:

{
  "location": "http://<url>/get_song/Tvsjkyaodufagbn6oztvhgahmna",
  "title": "Christmas (Baby Please Come Home)",
  "creator": "Michael Bublé",
  "album": "Christmas (Deluxe Special Edition)",
  "trackNum": 9,
  "duration": "187000",
  "annotation": "year: 2011",
  "image": "http://lh3.googleusercontent.com/ozTe98yAyQ9pvyHxovQ5TbuAXPYtKB7h-CeFhALHf4dNnyh1oLdHZoJs_6mw2OACg7rxgUjH",
  
  // Add these:
  "storeId": "Tapfed6i3y5vg3jfqo7l4c3afre",
  "albumId": "Bco37q2mn5dckgo2mdigur5vhmq",
  "artistId": [
    "Adldfhipxacjxjcyn7qw7l5dvqa"
  ],
  "trackId": "Tvsjkyaodufagbn6oztvhgahmna"
}