Add history and download information into the API

History information is available from the contents API together with other metadata.  For example, if you go to http://localhost:6543/contents/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7, you'll get the usual metadata like:

```
"id": "e79ffde3-7fb4-4af3-9ec8-df648b391597",
"subject": "Mathematics and Statistics,Science and Technology",
"title": "College Physics",
"version": "1.7",
```

But now you'll also get this:

```
"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"
            }
        }
    ]
```

That's the history information.

As for the downloads, you'll need to get it from another API.  For example, if you go to http://localhost:6543/exports-allowable-types/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7, you'll get this:

```
[
    {
        "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"
        }
    ]
```

This implementation is based on http://mountainbunker.org/~ew2/rewrite/OpenStaxCNX_DesignRevisions_v2.pdf (page 7 and 8)

Requirements and discussion #80