Facia tool: groups within a collection

Collection can optionally contain `groups`, specified in config.json, e.g.
```
{
  "displayName": "Top Stories",
  "id": "uk/news/top-stories",
  "groups": ["major", "minor", "other"]
}
```
Groups renders in the tool as below. The relevant `group` property is added to items dragged into that group (though not yet persisted by facia-tool api):

![screenshot from 2013-10-02 16 16 59](https://f.cloud.github.com/assets/1147913/1254459/4aec6ef6-2b76-11e3-9028-68e43d37e217.png)