V0.4 peanuts store sync events

I've implemented a "sync" event for `hoodie.store.on`. It gets triggered on each object whose local changes have been pushed.

```js
hoodie.store.on('sync', markObjectAsSynced )
hoodie.store.on('sync:document', function(doc) {} )
hoodie.store.on('sync:$message:123', callProgressCallbackOnTask )
```

@svnlto