Handle 'HATEOAS' configuration setting.
Links added to payload if HATEOAS is enabled.
JSON renders both lists and dicts.
XML renders both lists and dicts.
Homepage returns a 404 when HATOEAS is disabled.
HATEOAS enabled by default.
Tests for optional HATEOAS.
Documentation for optional HATEOAS.
Optional HATEOAS
Merge branch 'optional_hateoas' into develop

HATEOAS can be disabled both at API and/or resource level. When HATEOAS
is disabled response payloads have a different structure. The resource
payload is a simple list of items:

    $ curl -i http://eve-demo.herokuapp.com/people/
    HTTP/1.0 200 OK

    [
        {
            "firstname": "Mark",
            "lastname": "Green",
            "born": "Sat, 23 Feb 1985 12:00:00 GMT",
            "role": ["copy", "author"],
            "location": {"city": "New York", "address": "4925 Lacross Road"},
            "_id": "50bf198338345b1c604faf31",
            "updated": "Wed, 05 Dec 2012 09:53:07 GMT",
            "created": "Wed, 05 Dec 2012 09:53:07 GMT",
            "etag": "ec5e8200b8fa0596afe9ca71a87f23e71ca30e2d",
        },
        {
            "firstname": "John",
            ...
        },
    ]

As you can see the ``_links`` element is also missing from list items.
The same happens to individual item payloads:

    $ curl -i http://eve-demo.herokuapp.com/people/50acfba938345b0978fccad7/
    HTTP/1.0 200 OK

    {
        "firstname": "John",
        "lastname": "Doe",
        "born": "Thu, 27 Aug 1970 14:37:13 GMT",
        "role": ["author"],
        "location": {"city": "Auburn", "address": "422 South Gay Street"},
        "_id": "50acfba938345b0978fccad7"
        "updated": "Wed, 21 Nov 2012 16:04:56 GMT",
        "created": "Wed, 21 Nov 2012 16:04:56 GMT",
        "etag": "28995829ee85d69c4c18d597a0f68ae606a266cc",
    }

When HATEOAS is disabled the API entry point (the home page) will return
a ``404 Not Found``, since its only usefulness would be to return a list
of available resources, which is the standard behavior when HATEOAS is
enabled.