Add initial read-only REST API endpoints for snapshots

[westonruter]

Examples:

Get all draft snapshots authored by user with username "jsmith":

http://src.wordpress-develop.dev/wp-json/wp/v2/customize_snapshots?status=draft&author=jsmith

Get a snapshot by a UUID:

http://src.wordpress-develop.dev/wp-json/wp/v2/customize_snapshots?status=any&slug=fcc0a3f0-2d6d-4e4b-906b-46025857b07a

An individual item will look like the following (take note of content):

{
  "id": 6030,
  "date": "2016-07-23T06:11:22",
  "date_gmt": null,
  "guid": {
    "rendered": "http://src.wordpress-develop.dev/?customize_snapshot_uuid=fcc0a3f0-2d6d-4e4b-906b-46025857b07a"
  },
  "modified": "2016-07-23T06:11:22",
  "modified_gmt": null,
  "slug": "fcc0a3f0-2d6d-4e4b-906b-46025857b07a",
  "type": "customize_snapshot",
  "link": "http://src.wordpress-develop.dev/?customize_snapshot_uuid=fcc0a3f0-2d6d-4e4b-906b-46025857b07a",
  "content": {
    "blogname": {
      "value": "WordPress Coool"
    }
  },
  "author": 4,
  "_links": {
    "self": [
      {
        "href": "http://src.wordpress-develop.dev/wp-json/wp/v2/customize_snapshots/6030"
      }
    ],
    "collection": [
      {
        "href": "http://src.wordpress-develop.dev/wp-json/wp/v2/customize_snapshots"
      }
    ],
    "about": [
      {
        "href": "http://src.wordpress-develop.dev/wp-json/wp/v2/types/customize_snapshot"
      }
    ],
    "author": [
      {
        "embeddable": true,
        "href": "http://src.wordpress-develop.dev/wp-json/wp/v2/users/4"
      }
    ],
    "version-history": [
      {
        "href": "http://src.wordpress-develop.dev/wp-json/wp/v2/customize_snapshots/6030/revisions"
      }
    ],
    "wp:attachment": [
      {
        "href": "http://src.wordpress-develop.dev/wp-json/wp/v2/media?parent=6030"
      }
    ],
    "curies": [
      {
        "name": "wp",
        "href": "https://api.w.org/{rel}",
        "templated": true
      }
    ]
  }
}

Fixes #29.

[westonruter]

See https://core.trac.wordpress.org/ticket/38900