Create an adapter source

The `AdapterSource` is a pseudo-source that transforms and provides the data coming from another source. It allows users to use widgets requiring a certain data type with sources that provide data in another.

An usage example:

```javascript
sources: {
  (...)
  rssSource: {
    className: 'Dashboard.RssSource',
    args: {
      feedUrl: "http://rss.slashdot.org/Slashdot/slashdot"
    }
  }
},

(...)

widgets: [
  (...)
  {
    pos: [2, 2],
    widget: 'Dashboard.RssWidget',
    source: 'rssSource',
    args: { rotatePeriod: 15000 }
  },
  {
    pos: [2, 3],
    widget: 'Dashboard.TextWidget',
    source: 'Dashboard.AdapterSource',
    sourceArgs: {
      source: { className: 'rssSource' },
      transform: function(data) { return data.entries[0].title; }
    }
  }
]
```