Add a RSS widget and source.

The widget and source can be tested with the following widget config:

```
  {
    pos: [2, 1],
    widget: 'Dashboard.RssWidget',
    source: 'Dashboard.RssSource',
    sourceArgs: {
      feedUrl: "http://rss.slashdot.org/Slashdot/slashdot"
    }
  }
```