Facia tool: groups within a collection

In config.json, a collection can have an optional `groups` property:
```
{
  "displayName": "Top Stories",
  "id": "uk/news/top-stories",
  "groups": ["small", "medium", "large"]
}
```
When an article is dragged into a group, the zero-based index of the group will be saved as a property on the item in the collection (note: the group prop isn't yet persisted by the facia-tool API):
```
{
  "live" : [ 
    {
      "id" : "world/2013/oct/02/us-shutdown-debt-ceiling-crisis-republicans",
      "group": 2
    }
   ...etc
```
The tool maintains the overall order of the collection, rendering groups as separate draggable lists (so that they can't interleave): 

![fronts-tool](https://f.cloud.github.com/assets/1147913/1261083/a9340ed0-2c23-11e3-89c0-0456a63374bf.png)