Fixed description parsing bug

We're looking into using InstapaperKit for a project here at @Lickability, and we noticed that it seems the Instapaper API has been updated to return bookmark descriptions keyed by `description` instead of `descr`. We made that change to `IKDeserializer` and started getting descriptions back.

From the [Instapaper API Documentation](http://www.instapaper.com/api/full):
```json
 {"type":"bookmark", "bookmark_id":1234, "url":"http:\/\/www.example.com\/page1.html", "title":"Example page 1", "description":"An example page."},
 {"type":"bookmark", "bookmark_id":1235, "url":"http:\/\/www.example.com\/page2.html", "title":"Example page 2", "description":"Another example page."},
 {"type":"bookmark", "bookmark_id":1236, "url":"http:\/\/www.example.com\/page3.html", "title":"Example page 3", "description":"Yet another example page."}]
```