"dream snaps"

These cause the presser to always fetch the latest item from a tag, specified by `snapUri`. They have this form in a collection.json file:
```
{
  id: "snap/1415967038995",
  frontPublicationDate: 1415967046702,
  meta: {
    snapType: "latest",
    snapUri: "technology",
    customKicker: "Latest from Technology",
    showKickerCustom: true
  }
}
```

This PR also unifies the spec of default snaps (ie a link to a page), as:
```
{
  id: "snap/1415967005324",
  frontPublicationDate: 1415967049488,
  meta: {
    snapType: "link",
    snapUri: "http://www.theguardian.com/fashion",
    headline: "Fashion news, advice and pictures | The Guardian"
  }
},
whereas previously they used a single `href` property instead of the snap* properties.