Adding event sources everywhere

It's somewhat primitive, but it works.

It looks like this when logged:

```
Info: -[MSAppContext zincRepo:didReceiveEvent:] : DOWNLOAD-COMPLETE: 'url'='http://zinc.mindsnacks.com/com.mindsnacks.lessons/manifests/english-french-lesson-pack-28.json.gz' 'source'='{
    class = ZincDownloadCompleteEvent;
    function = "+[ZincDownloadCompleteEvent downloadCompleteEventForURL:size:]";
    line = 213;
    object = 0xac85bc;
}' 'size'='0'
```