Show an empty row when there is no data

At the moment, if the collection in the datagrid is empty you've got the result
![Screen Shot 2013-04-03 at 18 43 22](https://f.cloud.github.com/assets/445628/335224/ce9de6fa-9c86-11e2-8f94-5d8dc032687a.png)

I've created an empty row view which display a row with a single column - with a colspan -
![Screen Shot 2013-04-03 at 18 42 43](https://f.cloud.github.com/assets/445628/335238/eec083a2-9c86-11e2-9fdd-e1877dd5d417.png)

You can configure the text inside the empty row
```javascript
var grid = new Backgrid.Grid({
  columns: columns,
  collection: territories,
  emptyText: "no data"
});
```
![Screen Shot 2013-04-03 at 18 42 24](https://f.cloud.github.com/assets/445628/335245/23d7d464-9c87-11e2-9eb1-c52db5ace994.png)


I hope you like the feature & the code

David