Include posted time in API

Currently, items in the API (for example the friend shake) don't include the time when the item was posted. This would be helpful when showing mlkshk items ordered by time alongside other sources.

For example, with a timestamp as posted an item may appear as:

{
    "description": null,
    "height": 720,
    "name": "old_woman_gun_pills.jpg",
    "original_image_url": "http://mlkshk.com/r/XPX",
    "permalink_page": "http://mlkshk.com/p/XPX",
    "posted": "2011-03-12T12:04:19Z",
    "title": "Hurricane Grandma.jpg",
    "user_id": 533,
    "user_name": "drewtoothpaste",
    "width": 480
}

It'd be great if the time were present as a UTC time in ISO 8601 format (that's what I usually see in JSON APIs). That is:

timestamp = created.replace(microsecond=0, tzinfo=None).isoformat() + 'Z'

where created is a datetime.datetime.