Add playlist capability

[dmreagan]

Add a simple playlist creator/editor and display.

User copies collection ID from collection details page and adds it to an array in the playlist create/edit page.

{
  "metadata": {
    "name": "playlist1",
    "description": "description1",
  },
  "collection-ids": [
    "id1",
    "id2",
    "id3",
  ]
}

[dmreagan]

{
  "metadata": {
    "name": "playlist1",
    "description": "description 1",
  },
  "collections": [
    {
      "id": "collection1",
      "duration": 30,
    },
    {
      "id": "collection2",
      "duration": 45,
    }
  ]
}

[dmreagan]

For timer logic, use the AngularJS $interval service. See examples from the screensaver and img-slideshow.

[guangchen]

Implemented the playlist feature.