Add gallery support

[SamWhited]

Add support for photo galleries in place of individual cover photos. Maybe add plugins for common photo api's. Generate gallery JSON and load that to render gallery.

• page.cover_type (nil|gallery)
• page.flickr_album (id)
• page.500px_album (id)
• page.photos (src array)
• page.thumbnail (animate|src)
• page.show_in_gallery (use to exclude src from site wide or category galleries; page level galleries are automatically excluded)
• page.gallery_speed (seconds)

The page.src attribute can still be used and will be an alt image if the gallery can't be loaded for some reason. Also add a cover photos gallery view to categories and a site wide photos gallery to special pages.

Gallery JSON example:

{
    "images": [{
        "src": "...",
        "thumbnail": "...",
        "caption": "...",
        "title": "...",
        "alt": "..."
    },
    "flickr api URI",
    "Other api URI",
    "Etc."
    ]
}

[SamWhited]

I don't think I'll bother with Flickr or 500px API support, or with making individual posts able to have galleries. Simply a global gallery page and JSON files, and category gallery pages / JSON files.