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/). I've added `hide_from_robots` value to the response as well as `x-robots-tag: noindex` to the header if noindex is enabled.

Example response before the change:
-----------------------------------------------

    GET http://reddit.local/user/test123/about.json

    HTTP/1.0 200 OK
    Server: PasteWSGIServer/0.5 Python/2.7.3
    Date: Sat, 01 Nov 2014 02:39:04 GMT
    content-type: application/json; charset=UTF-8
    x-ua-compatible: IE=edge
    content-length: 257
    x-frame-options: SAMEORIGIN
    x-content-type-options: nosniff
    x-xss-protection: 1; mode=block
    x-reddit-pagecache: miss
    x-ratelimit-remaining: 297
    x-ratelimit-used: 3
    x-ratelimit-reset: 56

    {
      "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

    HTTP/1.0 200 OK
    Server: PasteWSGIServer/0.5 Python/2.7.3
    Date: Sat, 01 Nov 2014 02:41:17 GMT
    content-type: application/json; charset=UTF-8
    x-ua-compatible: IE=edge
    x-robots-tag: noindex, nofollow
    content-length: 256
    x-frame-options: SAMEORIGIN
    x-content-type-options: nosniff
    x-xss-protection: 1; mode=block
    x-reddit-pagecache: miss
    x-ratelimit-remaining: 298
    x-ratelimit-used: 2
    x-ratelimit-reset: 523

    {
      "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"
      }
    }