Include user's index preference in account API

3rd party sites allow indexing of user profiles. Perhaps they will implement `noindex` if the user's preference is easily accessible. This is an implementation of this [request.](https://www.reddit.com/r/ideasfortheadmins/comments/2ktryi/allow_reddit_clients_to_respect_users_noindex/)

Example response before the change:
-----------------------------------------------

    GET http://reddit.local/user/test123/about.json
    {
      "kind": "t2",
      "data": {
        "name": "test123",
        "is_friend": false,
        "created": 1414750716,
        "created_utc": 1414721916,
        "link_karma": 1,
        "comment_karma": 0,
        "is_gold": false,
        "is_mod": false,
        "has_verified_email": false,
        "id": "pc"
      }
    }

Example Response after the change:
-----------------------------------------------

    GET http://reddit.local/user/test123/about.json
    {
      "kind": "t2",
      "data": {
        "name": "test123",
        "is_friend": false,
        "created": 1414750716,
        "hide_from_robots": true,
        "created_utc": 1414721916,
        "link_karma": 1,
        "comment_karma": 0,
        "is_gold": false,
        "is_mod": false,
        "has_verified_email": false,
        "id": "pc"
      }
    }