Implement GET on root API

Should return top level API resource.

Relevant bits from the API Spec:

API
----

An API resource is returned at the root URL (or entry point) to the API. From
here all versions and subsequent resources are discoverable.

Usage
^^^^^^

======= ============= =====================
Verb Path Response
======= ============= =====================
GET / Get the API resource
======= ============= =====================

Fields
^^^^^^^

type
    The type of this resource, i.e. api
name
    The name of the API, e,g, openstack.ironic.api
description
    Some information about this API
versions
    A link to all the versions available in this API
default_version
    A link to the default version used when no version is specified in the URL
    or in the content-type

Example
^^^^^^^^

JSON structure of an API::

  {
    "type": "api",
    "name": "openstack ironic API",
    "description": "foobar",
    "versions": {
      "links": [{
          "rel": "self",
          "href": "http://localhost:8080/api/versions/"
        }, {
          "rel": "bookmark",
          "href": "http://localhost:8080/api/versions"
        }
      ]
    },
    "default_version": {
      "id": "1.0",
      "type": "version",
      "links": [{
          "rel": "self",
          "href": "http://localhost:8080/api/versions/1.0/"
        }, {
          "rel": "bookmark",
          "href": "http://localhost:8080/api/versions/1.0/"
        }
      ]
    }
  }