History info and extra metadata API

- Only return history information up to the displayed version

This means that when you go to http://localhost:6543/contents/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7, you will see 1.6 and 1.7 in history
```
"history": [
    {
        "changes": "New version 1.7",
        "version": "1.7",
        "revised": "2013-08-31 21:07:20.342798+02",
        "publisher": {"website": null, "surname": null, "suffix": null, "firstname": "OpenStax College", "title": null, "othername": null, "email": "info@openstaxcollege.org", "fullname": "OpenStax College", "id": "e5a07af6-09b9-4b74-aa7a-b7510bee90b8"}
    },
    {
        "changes": "Updated something",
        "version": "1.6",
        "revised": "2013-07-31 21:07:20.342798+02",
        "publisher": {"website": null, "surname": null, "suffix": null, "firstname": "OpenStax College", "title": null, "othername": null, "email": "info@openstaxcollege.org", "fullname": "OpenStax College", "id": "e5a07af6-09b9-4b74-aa7a-b7510bee90b8"}
    }
]
```

But when you go to http://localhost:6543/contents/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.6, you will only see 1.6 in history

```
"history": [
    {
        "changes": "Updated something",
        "version": "1.6",
        "revised": "2013-07-31 21:07:20.342798+02",
        "publisher": {"website": null, "surname": null, "suffix": null, "firstname": "OpenStax College", "title": null, "othername": null, "email": "info@openstaxcollege.org", "fullname": "OpenStax College", "id": "e5a07af6-09b9-4b74-aa7a-b7510bee90b8"}
    }
]
```

- Move downloads information into extra API
- Add is_latest flag to the extra API

Instead of having a separate API for just the download, I'm creating an "extra" API for getting information about a collection or module that can change after it is published.

So if you go to http://localhost:6543/extra/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7, you'll get the downloads and the is latest flag:

```
{
    "downloads": [
        {
            "path": "/exports/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7.pdf",
            "filename": "college-physics.pdf",
            "details": "PDF file, for viewing content offline and printing.",
            "format": "PDF"
        }, {
            "path": "/exports/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7.epub",
            "filename": "college-physics.epub",
            "details": "Electronic book format file, for viewing on mobile devices.",
            "format": "EPUB"
        }, {
            "path": "/exports/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7.zip",
            "filename": "college-physics.zip",
            "details": "An offline HTML copy of the content.  Also includes XML, included media files, and other support files.",
            "format": "Offline ZIP"
        }
    ],
    "is_latest": true
}
```