adds code for creating interactives, puts it in sprint 2

@Dantemss to review, @philschatz to use after merged -- same way as do existing sprint2 setup, now it'll just give the following on the API call:

```
{
  total_count: 2,
  items: [
    {
      id: 1,
      type: "reading",
      task_plan_id: null,
      opens_at: "2014-10-15T17:16:59.657Z",
      is_shared: false,
      content_url: "http://archive.cnx.org/contents/3e1fc4c6-b090-47c1-8170-8578198cc3f0@8.html"
    },
    {
      id: 2,
      type: "interactive",
      task_plan_id: null,
      opens_at: "2014-10-15T17:16:59.723Z",
      due_at: "2014-10-22T17:16:59.723Z",
      is_shared: false,
      content_url: "http://connexions.github.io/simulations"
    }
  ]
}
```